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S 

Bulletin  266  March,  1925         ^2 

QJunnrrttrut  A^rtrultural  ^£xpnmmt  ^tattu« 

'^^ta  Haopn,  (Cottn^rttrut 


The    Improvement    of    Naturally    Cross- 
Pollinated  Plants  by  Selection  in 
Self-Fertilized  Lines 

I.     THE  PRODUCTION  OF   INBRED  STRAINS 
OF  CORN 


D.  F.  Jones 
P.  C.  Mangelsdorf 


The  Bulletins  of  this  Station  are  mailed  free  to  citizens  of  Connecticut 
who  apply  for  them,  and  to  other  applicants  as  far  as  the  editions  permit. 


CONNECTICUT  AGRICULTURAL  EXPERIMENT  STATION 

OFFICERS  AND  SIAFF 
March  1925. 


BOARD  OF  CONTROL. 
His  Excellency,  John  H.  Trumbull,  ex-officio,  President. 

Charles  R.  Treat,  Vice  President Orange 

George  A.  Hopson,  Secretary Mount  Carmel 

Wm.  L.  Slate,  Jr.,  Director  and  Treasure  r New  Haven 

Joseph  W.  Alsop Avon 

Elijah  Rogers Southington 

Edward  C.  Schneider Middletown 

Francis  F.  Lincoln Cheshire 

STAFF. 
E.  H.  Jenkins,  Ph.D.,  Director  Emeritus. 


Administration. 


Chemistry. 

Analytical  Laboratory. 


Wm.  L.  Slate,  Jr.,  B.Sc,  Director  and  Treasurer. 
Miss  L.  M.  Brautlecht,  BooRkeeper  and  Librarian. 
Miss  J.  V.  Berger,  Stenographer  and  Bookkeeper. 
Miss  Mary  E.  Bradley,  Secretary. 
William  Veitch,  In  charge  of  Buildings  and  Grounds. 

E,  M.  Bailey,  Ph.D.,  Chemist  in  Charge. 

R.  E.  Andrew,  M.A.  1 

C.  E.  Shepard  I 

Owen  L.  Nolan  [  Assistant  Chemists. 

Harry  J.  Fisher,  A.B. 

W.  T.  Mathis  J 

Frank  C.  Sheldon,  Laboratory  Assistant. 

V.  L.  Churchill,  Sampling  Agent. 

Miss  Mabel  Bacon,  Stenographer. 


Biochemical 
Laboratory. 

Botany. 


Entomology*. 


T.  B.  Osborne,  Ph.D.,  Sc.D.,  Chemist  in  Charge. 


G.  P.  Clinton,  Sc.D.,  Botanist  in  Charge. 

E.  M.  Stoddard,  B.S.,  Pomologist. 

Miss  Florence  A.  McCormick-,  Ph.D.,  Pathologist. 

Willis  R.  Hunt,  M.S.,  Graduate  Assistant. 

G.  E.  Graham,  General  Assistant. 

Mrs.  W.  W.  Kelsey,  Secretary. 

W.    E.    Britton,    Ph.D.,    Entomologist   in   Charge;   State    Ento- 
mologist. 
B.  H.  Walden,  B.Agr. 
M.  P.  Zappe,  B.S. 
Philip  Garman,  Ph.D. 

Roger  B.  Friend,  B.S.,  Graduate  Assistant. 
John  T.  Ashworth,  Deputy  in  Charge  of  Gipsy  Moth   Work, 
R.  C.  Botsford,  Deputy  in  Charge  of  Mosquito  Elimination. 
Miss  Gladys  M.  Finley,  Stenographer. 


Assistant  Entomologists. 


Forestry. 


Walter  O.  Filley,  Forester  in  Charge. 
A.  E.  Moss,  M.F.,  Assistant  Forester. 
H.  W.  HiCOCK,  M.F.,  Assistant  Forester. 
Miss  Pauline  A.  Merchant,  Stenographer. 


Plant  Breeding. 


Donald  F.  Jones,  S.D.,  Geneticist  in  Charge. 
P.  C.  Mangelsdorf,  M.S.,  Graduate  Assistant. 


Soil  Research. 


M.  F.  Morgan,  M.S..  Investigator 

George  C.  Scarseth,  B.S.,  Graduate  Assistant. 


Tobacco  Sub-station 
at  Windsor 


-,  In  Churge. 


N.  T.  Nelson,  Ph.D..  Plant  Physiologist. 


The  Wilson  H.  Lee  Co. 


CONTENTS. 

Page 

The  effect  of  inbreeding  upon  corn 353 

Result  of  crossing 361 

An  interpretation  of  hybrid  vigor 364 

The  transitory  nature  of  hybrid  vigor 369 

Inbreeding  after  crossing 371 

The  attainment  of  complete  homozygosity 374 

Mutations  in  corn 375 

The  value  of  inbreeding 377 

Possibility  of  obtaining  vigorous  inbred  strains 380 

Selection  in  self-fertilized  lines 3S2 

Method  of  pollination 385 

Selection  of  ears  for  planting 385 

Elimination  of  self-fertilized  lines 386 

The  production  of  abnormalities 390 

The  approach  to  uniformity  and  constanc}^ 399 

Differences  in  the  selected  lines 401 

Susceptibility  to  disease 404 

Criterions  of  selection 410 

Classification  of  selected  lines 411 

Correlation  between  the  first  and  last  generations 412 

Limiting  factors 415 

Conclusion 417 


SUMMARY. 

The  results  of  previous  investigations  on  inbreeding  corn  are 
reviewed  to  show  the  development  of  the  method  of  selection  in 
self -fertilized  lines. 

Four  varieties  of  corn  have  been  self -fertilized  and  selected  for 
five  generations.  Eighty-six  lines  were  started  and  twenty  of 
these  were  lost  or  discarded. 

The  method  of  procedure  was  to  grow  three  progenies  in  each 
line  and  self -pollinate  five  of  the  most  desirable  appearing  plants 
in  the  best  progeny  each  year. 

A  large  number  of  clear-cut  recessive  abnormalities  appeared 
during  the  course  of  the  inbreeding.  In  all  except  one  case  these 
were  eliminated  by  the  fifth  generation. 

No  significant  difference  in  yield  was  found  between  segregating 
and  non-segregating  progenies  in  lines  showing  recessive  abnormal- 
ities in  the  previous  generation.  Also  lines  having  recessive 
abnormalities  at  the  start  showed  no  greater  reduction  in  yield 
during  the  five  generations  than  lines  that  were  free  from  them 
throughout  the  experiment. 

All  lines  showed  a  marked  reduction  in  yield  and  a  slowing  down 
of  the  rate  of  growth.  Although  great  differences  were  shown,  no 
lines  were  as  productive  as  the  original  variety.  No  appreciable 
correlation  was  found  between  the  characters  of  the  seed  ear, 
weight  of  seed,  size  of  seedling,  or  the  appearance  of  the  plants  at 
pollinating  time  and  the  production  of  grain  in  the  same  genera- 
tion. 

Some  correlation  in  certain  characters  was  found  between  the 
first  and  last  generations,  particularly  in  height  of  plant  and  in 
per  cent,  of  moldy  ears.  Less  association  was  shown  in  amount 
of  tillering  and  in  smut  infection,  while  in  productiveness  practically 
no  relation  was  found,  showing  that  good  and  poor  yielding  strains 
may  come  from  productive  or  unproductive  plants  at  the  start. 


THE  IMPROVEMENT  OF  NATURALLY  CROSS-POLLI- 
NATED PLANTS  BY  SELECTION  IN  SELF-FER- 
TILIZED LINES. 

I.     The  Production  of  Inbred  Strains  of  Corn. 

D.  F.  JONES  and  p.  c.  mangelsdorf 

The  improvement  of  naturally  self -fertilized  plants,  particularly 
the  small  grains,  has  gone  steadily  forward  following  the  develop- 
ment of  effective  methods  of  procedure.  In  contrast  to  the  older 
methods  of  mass  selection  based  upon  appearances,  stands  the 
system  of  individual  plant  selections  chosen  on  the  basis  of  the 
performance  of  their  progeny,  as  worked  out  by  Louis  de  Vilmorin 
in  1856  and  later  appHed  by  Hjalmer  Nilsson  in  1891  at  Svalof  in 
Sweden  and  by  W.  H.  Hays  at  the  Minnesota  Agricultural  Experi- 
ment Station  in  1892.  Although  the  early  methods  of  applying 
the  progeny  performance  test  involved  much  unnecessary^  effort, 
the  principle  was  sound  and  its  extensive  application  has  resulted 
in  a  large  number  of  valuable  new  varieties  of  important  crop 
plants,  notably  wheat  and  cotton.  The  theoretical  soundness  of 
this  procedure,  first  applied  in  an  empirical  way,  was  later  fully 
established  by  the  re-discovery  and  demonstration  of  Mendel's 
Law,  which  postulates  that  a  large  part  of  inherited  variability  is 
due  to  the  recombination  of  stable  units.  This  led  directly  to 
Johannsen's  genotype  conception  of  organisms  which  appear 
alike  but  breed  differently  and  those  which  are  themselves  diverse 
but  give  similar  offspring. 

The  improvement  of  naturally  cross-fertilized  plants,  reproduced 
by  seeds,  is  in  no  such  satisfactory  situation.  The  variation 
brought  about  by  Mendelian  recombination  makes  it  very  difficult 
to  have  any  adequate  control  over  the  heredity  when  inter- 
pollination  is  continually  going  on.  Moreover,  intensive  selection 
for  particular  characters  often  results  in  decreasing  the  niimber 
of  hybrid  combinations  and  this,  like  all  other  forms  of  inbreeding, 
brings  about  a  reduction  in  vigor.  Any  advantage  which  might 
come  about  from  the  concentration  of  desirable  germplasm  is 
offset  by  the  loss  of  growth  due  to  consanguinity. 

Com,  a  monoecious  plant  and  wind  pollinated,  is  almost  com- 
pletely cross-fertilized  in  every  generation.  This  mode  of  pollina- 
tion has  brought  about  a  condition  in  which  a  continuation  of  the 
same  degree  of  germinal  heterogeniety  is  necessary  to  maintain 
full  vigor.  The  experimental  results  of  inbreeding  and  crossing 
and  their  theoretical  interpretation  show  clearly  why  the  methods 
aimed  at  the  improvement  of  com  in  the  past  have  been  largely 
fruitless.  Formerly  the  selection  practiced  with  this  plant  was 
largely  based  upon  the  appearance  of  the  mature  ear.  Investiga- 
tion has  shown  that  com  has  now  been  brought  to  such  a  high 
plane  of  development  that  the  correlation  between  the  appearance 

(349) 


350 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


of  the  seed  and  the  productiveness  of  the  crop  grown  from  that 
seed  is  very  low;  so  low  in  fact  that  it  is  often  possible  to  get  as 
good  results  from  planting  the  poorest  looking  ears  to  be  found  in 
a  field  as  from  the  choicest  specimens.  This  is  due  to  the  fact 
that  hybrid  combinations  of  hereditary  factors  which  make 
possible  high  production  can  not  be  transmitted  intact  and  there- 
fore the  offspring  of  any  exceptional  individual  can  not  all  be 
equally  productive. 

An  early  appreciation  of  this  situation  following  the  application 
of  experimental  methods  to  the  study  of  com  breeding  led  to  the 
ear-to-row  system  in  which  selection  was  based  on  the  performance 


Figure  16.  The  seed  from  these  large  and  small  ears  yielded  the  same. 
Their  difference  in  size  is  due,  not  to  heredity,  but  to  the  place  where  the 
plants  that  produced  them  happened  to  grow,  one  lot  in  a  good,  the  other 
in  a  poor  situation.  This  shows  the  complete  lack  of  correlation  in  this 
case  between  the  appearance  of  the  seed  ears  and  their  performance. 

of  the  progeny  instead  of  the  appearance  of  the  seed  parents. 
Although  the  progenies  differed  markedly  in  yield  those  above  the 
average  failed  to  maintain  their  high  production  in  later  genera- 
tions. 

In  1908  G.  H.  ShuU  outlined  a  method  of  com  breeding  radically 
different  from  any  previously  followed.  In  this  he  called  attention 
to  the  large  number  of  germinally  different  types  which  exist  in 
every  field  of  com  and  suggested  that  these  cotild  be  separated  out 


INTRODUCTION  351 

by  inbreeding.  Although  vigor  was  lost  by  this  process  this  was 
to  be  regained  by  crossing  inbred  strains  and  utilizing  only  the  first 
following  generation  in  which  hybrid  vigor  is  at  its  maximum. 
East  also  advocated  the  same  method  and  reached  the  same  con- 
clusions as  to  the  importance  of  hybrid  vigor,  as  the  result  of 
independent  observations  on  the  effects  of  inbreeding  and  hybrid- 
ization. The  crossing  of  different  varieties  of  com  had  been 
advocated  long  before  this  by  Beal  at  the  Michigan  Agricultural 
Experiment  Station,  and  Morrow,  Gardner  and  McCluer  at 
Illinois.  Two  important  contributions  to  methods  for  com  im- 
provement were  made  by  Shull  and  East.  One  was  making  clear 
the  complex  germinal  constitution  of  a  variety  in  a  cross-fertilized 
plant  such  as  com  and  the  way  in  which  the  composition  of  any 
particular  individual  is  masked  by  hybrid  vigor.  The  other  was 
in  showing  that  the  maximiim  degree  of  hybrid  vigor  could  be 
secured  by  first  reducing  the  plants  to  homozygosity  and  then 
crossing,  thereby  bringing  about  the  greatest  number  of  hybrid 
combinations  of  hereditary  units.  Both  East  and  Shull  con- 
sidered hybrid  vigor  as  a  physiological  stimulus  resulting  from  the 
condition  of  hybridity  itself,  differing  from  the  specific  action  of 
individual  hereditary  factors.  For  this  reason  they  stressed  the 
importance  of  securing  the  maximum  effect  of  hybrid  vigor.  The 
more  important  service  of  inbreeding  in  automatically  eliminating 
abnormalities  and  serious  weaknesses  and  in  making  possible  the 
detection  and  isolation  of  the  potentially  most  valuable  germ- 
plasm  was  not  fully  appreciated  at  first  by  those  who  attempted  to 
apply  this  method  to  com  improvement.  For  that  reason  the  full 
utilization  of  the  pure  line  principle  was  delayed  until  hybrid  vigor 
was  shown  to  be  merely  the  expression  of  dominant  hereditary 
factors.  This  brought  out  clearh^  and  forcefully  the  great  value  of 
inbreeding  as  a  means  of  obtaining  the  finest  hereditary  material 
existing  in  a  cross-fertilized  plant  like  com  by  controlling  the 
inheritance  through  the  pollen  parent  as  well  as  through  the  seed 
parent,  and  fixing  this  in  such  a  way  that  it  would  not  be  lost. 
Following  up  this  line  of  attack  a  method  of  corn  improvement  was 
outlined  in  1920  under  the  general  title  of  "Selection  in  Self- 
fertilized  Lines.".*  It  is  here  proposed  to  review  the  results  of 
inbreeding  and  crossing  which  have  led  to  the  development  of  this 
method  and  show  how  inbreeding  can  best  be  applied  to  the  im- 
provement of  com  and  other  naturally  cross-fertilized  plants. 
As  the  application  of  this  method  is  still  in  progress  the  plan  is  to 
publish  the  results  in  a  series  under  the  general  heading  of  "The 
Improvement  of  Naturally  Cross-PoUinated  Plants  by  Selection 
in  Self -fertilized  Lines."  The  first  of  this  series,  submitted  in  the 
following  pages,  deals  only  with  the  detection  and  isolation  of 
desirable  hereditary  qualities  in  com,  that  is,  the  production  of 
inbred  strains  which  possess   either  in  visible  expression  or  in 

*Jour.  Agronomy,  12:77-100. 


352 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


potential  power  those  valued  characters  that  make  for  increased 
production.  Later  publications  are  planned  to  deal  with  the  test- 
ing and  utilization  of  inbred  strains  of  com  and  the  application  of 
the  same  principle  and  method  to  other  cross-fertilized  plants. 


Figure  17.  Two  inbred  strains  from  the  same  variety  that  have  been 
grown  side  by  side  for  eighteen  years.  The  difference  in  abiHty  to  stand 
erect  is  inherited. 


THE  EFFECT  OF  INBREEDING  UPON  CORN 


353 


The  Effect  of  Inbreeding  Upon  Corn. 

All  of  the  main  types  of  com  such  as  dent,  flint,  sweet,  pop  and 
flour  corn  have  been  inbred  by  self-fertilization  for  several  succes- 
sive generations.  The  results  have  been  the  same  in  general  for 
all  types.  Particular  attention  has  been  given  to  several  strains 
resulting  from  a  variety  of  Learning  grown  originally  in  central 
Illinois.  Inbreeding  was  started  by  Dr.  E.  M.  East  in  1905.  Four 
lines  descending  from  three  individual  plants  at  the  start  have  been 
continued  to  the  present  time  under  the  direction  of  Dr.  H.  K. 
Hayes  and  later  by  the  writers,  and  in  1923  they  had  been  inbred 
by  seventeen  successive  self-fertilizations.  The  results  obtained 
have  been  reported  from  time  to  time.  Particular  reference  is 
made  to  "Inbreeding  in  Com"  and  the  "Distinction  between 
Development  and  Heredity  in  Inbreeding"  by  East,  published  in 
the  report  of  the  Connecticut  Agricultural  Station  and  in  the 
American  Naturalist,  and  "Heterozygosis  in  Evolution  and  in 
Plant  Breeding"  by  East  and  Hayes  in  a  Bureau  of  Plant  Industry 
Bulletin.  Later  results  are  given  in  a  bulletin  of  the  Connecticut 
Agricultural  Station  under  the  title  of  "The  Effects  of  Inbreeding 
and  Crossbreeding  on  Development"  and  the  "Attainment  of 
Homozygosity  in  Inbred  Strains  of  Maize"  in  Genetics  by  the 
senior  writer.     As  the  method  of  selection  in  self-fertilized  lines 


Table  I. 

Yield  and  Height  of  Four  Inbred  Learning  Strains  of  Corn  Self-Fertilized 
Seventeen  Generations. 


Strain  A 

Strain  B 

Strain  C 

Strain  D 

No.  of 

Yield 

Height 

Yield 

Height 

Yield 

Height 

Yield 

Height 

Gen. 

Bu. 

Bu. 

Bu. 

Bu. 

Selfed 

per  Acre 

Inches 

per  Acre 

Inches 

per  Acre 

Inches 

per  Acre 

Inches 

0 

74.7 

117.3 

74.7 

117.3 

74.7 

117.3 

74.7 

117.3 

1 

42.3 

60.9 

60.9 

59.1 

2 

51.7 

59.3 

59.3 

95.2 

3 

35.4 

46.0 

59.7 

57.9 

4 

47.7 

63.2 

68.1 

80.0 

5 

26.0 

76.5 

25.4 

81.1 

41.3 

96.5 

27.7 

86 '.7 

6 

38.9 

7 

45.4 

85.0 

39.4 

41.8 

8 

21.6 

47.2 

83.5 

58.5 

88  ".6 

78.8 

96.0 

9 

30.6 

78 '.7 

24.8. 

25.5 

10 

31.8 

82.4 

32.7 

84.9 

19.2 

86.9 

32.8 

97.7 

11 

35.1 

79.7 

42.3 

78.6 

37.6 

83.8 

46.2 

103.7 

12 

24.5 

77.0 

27.2 

80.3 

20.4 

85.2 

49.6 

100.4 

13 

26.9 

85.5 

29.0 

83.7 

25.1 

80.6 

25.8 

85.3 

14 

23.6 

87.3 

38.3 

86.9 

36.3 

87.8 

35.2 

94.0 

15 

21.1 

85.4 

33.4 

89.9 

30.0 

98.2 

33.6 

99.6 

16 

17.6 

76.1 

24.6 

89.1 

25.3 

94.6 

29.8 

97.7 

17 

27.8 

91.7 

16.9 

88.9 

19.8 

88.4 

354  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 

has  been  the  direct  outgrowth  of  these  investigations  as  to  the 
effects  of  inbreeding,  a  brief  restime  of  the  results  obtained  to  date 
will  be  given  here. 

The  method  of  inbreeding  followed  in  the  earlier  experiments 
was  to  self -pollinate  a  ntmiber  of  plants  at  random  and  use  one 
of  these  as  the  progenitor  for  the  following  generation.  Such  a 
family  descending  from  a  single  self -fertilized  plant  in  each  genera- 
tion is  called  a  line  or  strain.  The  yield  of  grain  and  height  of  plant 
of  four  hnes  from  Learning  during  seventeen  successive  self- 
fertilized  generations  compared  to  the  non-inbred  variety  are  given 
in  Table  I.  The  four  lines  A,  B,  C,  D,  were  derived  at  the  start 
from  three  different  plants.  One  of  these  was  separated  in  the  third 
generation  into  two  lines,  B  and  C.  These  have  been  continued 
separately  since.  Other  lines  were  started  from  the  same  variety 
but  have  since  been  lost  on  account  of  failure  to  secure  self -polli- 
nated seed.  In  some  cases  this  loss  has  been  accidental,  but  for 
the  most  part  these  strains  were  maintained  previous  to  their 
extinction  with  great  difficulty  and  showed  a  much  greater  reduc- 
tion in  growth  and  vigor  than  the  other  strains  which  survived. 

Although  there  is  wide  variation  in  yield  of  grain  and  height  of 
plant  from  year  to  year  the  general  direction  is  downward.  After 
the  ninth  generation  size  and  productiveness  have  remained  on 
about  the  same  level.  The  original  variety  yielded  at  the  rate  of 
eighty-eight  bushels  per  acre  the  year  it  was  first  self-fertilized. 
In  1916  seed  of  the  same  variety  was  obtained  from  the  original 
source  and  grown  in  comparison  with  these  strains,  then  in  the 
ninth  or  tenth  generation.  On  account  of  its  change  to  a  new 
location  under  conditions  to  which  it  was  not  as  well  adapted  as  the 
inbred  strains,  which  had  been  grown  there  for  many  years,  no 
strict  comparison  can  be  made.  In  spite  of  their  possible  ad- 
vantage the  inbred  strains  were  only  from  one-half  to  one-third  as 
productive  and  were  also  noticeably  reduced  in  height. 

This  decrease  in  yield  which  results  from  a  reduction  in  size  of  all 
parts  of  the  plant  and  a  lessening  of  the  growth  rate  has  so  far 
been  the  universal  result  of  inbreeding  com  as  far  as  known  to  the 
writers.  Several  hundred  self-fertilized  strains  have  been  grown 
long  enough  to  bring  this  out  clearly.  Accompanying  the  lessening 
of  productiveness  and  growth  vigor  there  has  been  a  reduction  in 
variability.  From  a  variety  that  showed  the  usual  variation  in 
height,  color  of  silks,  glimies  and  leaf  sheaths,  number  of  ears, 
position  of  the  ear  and  other  details  in  all  parts  of  the  plants  there 
resulted  in  the  four  self -fertilized  lines  a  marked  uniformity  among 
all  of  the  plants  within  each  line.  This  similarity  in  type  became 
noticeable  in  the  earlier  generations  of  inbreeding,  and  after  seven 
or  eight  successive  self-fertilizations  every  plant  in  any  one  line 
was  as  much  like  every  other  plant  in  that  line  as  any  two  plants 
in  a  naturally  self-fertilized  species,  such  as  wheat  or  tobacco, 
from  seed  from  the  same  individual.     In  other  words,  the  vari- 


THE    EFFECT    OF    INBREEDING    UPON    CORN 


355 


ability  that  resulted  from  the  recombination  of  hereditary  factors 
was  in  time  eliminated. 


Figure  18.      Two  inbred  strains  from  the  same  variety  of  flint  corn,  one 
with  many  tillers  and  the  other  without  any. 


356  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 

Where  the  original  variety  had  some  plants  with  colored  silks 
and  others  with  uncolored,  some  of  the  lines  now  have  all  their 
plants  with  red  silks  while  in  others  all  the  silks  are  green.  In 
some  lines  the  foliage  on  all  the  plants  is  a  bright  glossy  green,  in 
others  a  dull  bluish  green.  All  the  plants  of  one  of  the  lines  re- 
main green  and  stand  firmly  erect  throughout  the  season  while  in 
other  lines  the  foliage  turns  yellow  towards  the  end  of  the  growing 
season  and  in  still  another  the  plants  frequently  go  down  on 
account  of  a  weak  root  system.  Differences  in  susceptibility  to 
smut  are  shown  in  these  four  strains  as  brought  out  in  table  II. 
In  every  detail  of  structure  of  the  plant,  including  tassel  and  ears, 
all  the  individuals  of  one  line  are  remarkably  alike  and  noticeably 
different  from  the  other  lines.  Some  of  these  differences  are  shown 
in  the  accompanying  illustrations,  figures  17,  18  and  19.  The  uni- 
formity within  the  line  and  the  differences  between  the  several 
lines  are  brought  out  statistically  in  tables  III  to  VI,  which  show 
the  height  of  plant,  length  of  ear,  n amber  of  nodes  and  rows  of 
grain  on  the  ear  for  the  original  variety  and  the  four  strains  derived 
from  this  variety. 

Table  II. 

Per  cent,  of  Plants  Showing  Smut  Infestation  in  Fonr  Inbred  Learning 
Strains. 

Strain        1917     1918     1919     1920     1921     1922     1923     Ave 

A  .3  .7  1.9  14.3  15.2  3.0  .0  5.1 

B  9.8  25.9  8.6  32.8  50.0  27.3  69.0  31.9 

C  .5  9.1  4.1  6.0  13.8  17.5  52.7  14.8 

D  .0  1.0  1.4  25.0  4.1  2.2  .7  4.9 

During  the  early  generations  of  self-fertilization  various  forms  of 
abnormalities  appeared.  The  most  frequent  of  these  are  seedlings 
wholly  or  partially  lacking  in  chloroph3dl,  various  types  of  striped 
plants,  golden  plants,  dwarfs,  plants  with  ears  showing  many 
poorly  developed  and  aborted  seeds,  and  others  with  sterile  tassels 
and  ears.  These  are  a  few  of  the  more  strikingly  aberrant  types. 
Some  of  these  are  able  to  produce  seed  and  when  self-fertilized 
come  true  to  their  abnormal  condition.  Others  are  wholly  in- 
capable of  reproduction  and  are  eliminated,  but  the  inbred  strains 
in  which  they  appear  may  continue  to  produce  them  regularly  as 
part  of  their  offspring  in  the  following  generations.  After  several 
generations  these  abnormalities  are  usually  no  longer  produced 
and  the  remaining  plants  are  all  normal  in  type  although  reduced 
in  size  and  in  rapidity  of  growth.  Many  of  the  abnormal  forms 
which  appear  in  large  numbers  in  the  inbred  families  are  occasion- 
ally seen  in  fields  of  com  which  have  never  been  artificially  self- 
fertilized.  Obviously,  inbreeding  is  not  responsible  for  their 
creation.  They  are  recessive  in  mode  of  inheritance;  that  is,  when 
crossed  with  other  plants  the  following  generation  is  all  normal 
but  the  abnormality  reappears  in  the  subsequent  generations. 


THE  EFFECT  OF  INBREEDING  UPON  CORN 


357 


K^^^^Hii^ 


Figure  19.  Differences  in  height  of  two  inbred  strains  from  the  same 
variety  self-fertilized  four  generations  and  selected  for  vigor  and  produc- 
tiveness but  not  for  height. 


358 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


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THE  EFFECT  OF  INBREEDING  UPON  CORN 


359 


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360 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


In  ordinary  fields  of  com  they  are  generally  kept  out  of  sight  by 
continual  crossing  with  normal  types  which  are  dominant.  Plants 
carrying  such  factors  for  abnormality,  when  self -fertilized,  produce 
them  in  approximately  one-fourth  of  their  progeny.  Some  of  the 
normal  plants  in  the  same  progeny  carry  the  abnormality  and  some 
do  not.  Sooner  or  later,  progenitors  are  used  which  do  not  carry 
any  of  these  striking  abnormalities,  after  which  they  cease  to 
appear. 

The  rate  at  which  reduction  in  growth  takes  place  and  the  final 
size  and  productiveness  of  the  several  lines,  after  the  reduction 
comes  to  an  end,  vary  in  different  lines.  Of  the  four  Learning 
strains  the  D  line  has  regularly  been  taller  and  larger  and  has 
yielded  more  than  the  others.  The  rate  of  reduction  has  been 
nearly  alike  in  all  of  the  four  lines  although  A  was  reduced  in  yield 
somewhat  more  quickly  than  any  of  the  others.     The  attainment 


Figure  20.  Comparative  production  of  a  variety  of  Learning  corn, 
two  inbred  strains  derived  from  this  variety,  and  their  first  generation 
hybrid.  Grown  in  adjoining  rows,  they  yielded  96,  32,  20  and  115  bushels 
per  acre  respectively. 


of  uniformity  may  also  proceed  at  a  different  rate,  depending  upon 
the  degree  of  heterozygosity  of  the  plant  chosen  as  progenitor. 
Some  strains  remain  variable  for  many  generations  while  others 
become  uniform  in  nearly  every  feature  after  a  few  generations  of 
self-fertilization. 

From  the  foregoing  facts  it  is  obvious  that  inbreeding  is  a  process 
of  sorting  out.  From  a  mixture  of  many  genetically  different 
individuals  all  varying  in  hereditary  composition  and  in  heterozy- 
gosity any  number  of  homozygous  lines  can  be  ultimately  obtained, 
each  differing  to  a  greater  or  less  degree  from  every  other.  A 
naturally  cross-fertilized  species  is  thus  changed  into  an  artifically 
self -fertilized  species.  In  uniformity  and  constancy  these  artific- 
ially inbred  plants  are  quite  comparable  to  naturally  self -fertilized 
species,  with  the  important  difference  that  in  com  they  are  mark- 
edly reduced  in  size  and  vigor. 


result  of  crossing 
Result  of  Crossing. 


361 


The  vigor  which  is  lost  by  inbreeding  is  at  once  restored  when 
two  self-fertiHzed  Hnes  descending  from  different  plants  at  the  start 


Figure  21.      Two  inbred  strains  and  their  first  generation  hybrid  show- 
ing differences  in  time  of  flowering. 

are  crossed.  This  is  shown  in  figure  20.  Here  the  ears  produced 
by  the  original  non-inbred  variety  are  shown  in  comparison  with 
the  ears  produced  by  two  Hnes  self -fertilized  12  generations  and  the 


362 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


first  generation  hybrid  between  these  two  Hnes.  An  equal  number 
of  plants  of  the  four  lots  were  grown  in  adjoining  rows  and  yielded 
96,  32,  20  and  115  bushels  per  acre  respectively.  A  comparison  of 
a  large  number  of  first  generation  crosses  between  inbred  strains 
derived  from  the  same  variety  showed  that  the  yield  of  the  hybrids 
was  increased  180  per  cent.,  height  of  plant  27,  length  of  ear  29, 
number  of  nodes  6,  and  rows  of  grain  on  the  ear  5  per  cent,  above 
the  average  of  their  inbred  parents.*  From  this  it  is  seen  that 
size  characters  such  as  height  of  plant  and  length  of  ear  are  affected 
more  noticeably  by  hybrid  vigor  than  the  number  of  parts,  such 
as  nodes  and  rows  of  grain  on  the  ear,  while  yield,  which  stmis  up 


Figure  22.  Representative  ears  of  three  inbred  strains  of  dent  corn 
and  two  first  generation  hybrids  resulting  from  the  crossing  of  the  two 
adjoining  types,  harvested    at    the  same  time  to  show  the  difference  in 

maturity. 

the  entire  growing  capacity  of  the  plant,  is  increased  more  than 
anything  else.  In  other  words  hybrid  vigor  has  much  the  same 
effect  as  favorable  environmental  factors.  Fertile  soil,  good 
season  and  careful  cultivation  influence  the  growth  of  the  com 
plant.  Under  these  conditions  corn  grows  taller,  the  ears  are 
larger  and  the  production  of  grain  is  much  greater  than  under 
the  less  favorable  conditions,  while  the  number  of  nodes  or  the 
rows  of  grain  on  the  ear  are  not  so  much  changed. 

*"The  effects  of  inbreeding  and  crossbreeding  upon  development." 
Connecticut  Agric.  Exper.  Station  Bull.  207. 


RESULT    OF    CROSSING 


363 


Another  noticeable  effect  of  crossing  inbred  strains  of  com  is 
that  of  hastening  the  time  of  flowering  and  maturing.  Figure 
21  shows  two  inbred  strains  in  which  the  tassels  are  just  beginning 
to  appear.  No  silks  are  out.  The  first  generation  hybrid  of 
these  two  strains  in  the  center  is  shedding  pollen  from  nearly  all 
of  the  tassels  and  the  silks  are  well  out  on  many  of  the  plants. 
Representative  ears  of  three  inbred  strains  and  first  generation 
hybrid  ears  resulting  from  the  cross  of  the  two  adjacent  strains  are 
pictured  in  figure  22.  All  were  picked  at  the  same  time  and  show 
the  greater  maturity  of  the  hybrid  ears. 

All  of  the  combinations  of  inbred  strains  have  shown  increased 


Figure  23.  A  first  generation  hybrid  showing  the  uniformity  in  height 
and  in  tassel  type.  The  two  inbred  parental  strains  are  in  the  adjoining 
rows  at  the  left. 


growth  and  yield  whether  the  parental  strains  come  from  the  same 
original  variety  or  from  different  varieties.  Some  combinations 
have  yielded  more  than  others.  A  few  have  been  better  than 
others  in  many  respects.  Crosses  between  strains  from  different 
varieties  have  not  been  conspicuously  better  than  crosses  within 
the  variety  although  no  extensive  test  of  this  point  has  been  made. 
Furthermore,  no  reliable  comparison  of  the  yield  of  the  hybrids 
with  the  original  variety  can  be  made  because  this  variety  is  not 
well  adapted  to  the  local  conditions  in  which  the  self-fertilized 


364  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 

lines  have  been  grown  for  many  years.  Kiesselbach  reports  the 
average  yield  of  seven  first  generation  hybrids  tested  two  years 
as  52  bushels  per  acre  in  comparison  with  42  bushels  for  the  original 
variety.  This  is  an  increase  of  24  per  cent.  The  highest  yielding 
hybrid  produced  59  bushels  or  an  increase  of  40  per  cent. 

The  most  noticeable  and  important  feature  of  the  first  generation 
hybrids  between  fixed  inbred  strains  is  the  even  gro^vth,  similarity 
in  size  and  structural  details  and  uniform  production  of  all  plants 
where  the  growing  conditions  are  equal.  This  is  shown  for  height 
of  plant  and  tassel  type  in  figure  23.  Barring  accident  every  plant 
is  like  every  other  plant.  They  grow  to  the  same  height.  All  ears 
are  borne  usually  at  the  same  node.  The  tassels  and  silks  appear 
at  the  same  time  and  the  plants  all  ripen  within  a  few  days  of  each 
other.  The  fact  that  every  plant  produces  a  good  ear  is  a  most 
important  factor  in  making  crosses  between  strains  so  productive. 
In  ability  to  yield  from  every  plant  and  in  uniformity  of  ripening, 
these  first  generation  com  hybrids  are  equal  to  any  naturally  self- 
fertilized  crop  such  as  wheat  and  tobacco  or  any  vegetatively 
propagated  plant  as  potatoes  and  sugar  cane.  Since  com  is  very 
susceptible  to  damage  by  unfavorable  weather  at  pollinating  time, 
the  uniformity  in  flowering  may  be  undesirable  particularly  in  those 
regions  where  hot  dry  weather  is  a  frequent  occurrence  at  this 
critical  time.  For  that  reason  some  other  method  of  utilizing 
inbred  strains  may  prove  to  be  more  practicable.  This  will  be 
considered  more  fully  in  later  publications.  It  is  sufficient  here  to 
point  out  that  in  these  first  generation  hybrids  we  have  a  new 
kind  of  corn  which  in  many  important  respects  is  radically  different 
from  the  mixtures  of  hybrids  of  varying  degrees  of  heterozygosity 
now  constituting  an  ordinary  field  of  com. 

An  Interpretation  of  Hybrid  Vigor. 

The  observations  of  gardeners  and  animal  husbandmen  have 
led  to  a  general  conviction  that  crossing  somewhat  different  but 
related  plants  or  animals  usually  results  in  a  greater  gro^\'th. 
Many  instances  of  this  phenomenon  of  hybrid  vigor,  in  which  the 
offspring  excel  both  parents  have  been  noted  in  the  higher  plants 
and  in  mammals,  birds,  insects  and  some  of  the  lower  forms  of 
animals.  Largei  size  or  more  rapid  growth  usually  results  when 
the  parents  are  visibly  different  in  some  respects  but  are  sufficiently 
related  to  produce  fertile  offspring.  Many  notable  cases  of  hybrid 
vigoi'  also  occur  in  wider  crosses  where  the  offspring  are  partially 
or  wholly  sterile.  This  is  well  illustrated  by  the  mule,  which  is 
sterile.  A  similar  wide  cross  in  plants  is  the  combination  of  the 
radish  and  cabbage  in  which  the  hybrid  makes  a  luxuriant  gro^^^h 
but  sets  no  seed.  Some  species  crosses  show  no  increased  vigor 
but  on  the  other  hand  may  be  extremely  weak.  East  and  Hayes 
have  given  several  illustrations  of  tobacco  hybrids  which  are 
barely  able  to  live  and  make  only  a  weak  growth.     Many  crosses 


AN    INTERPRETATION    OF    HYBRID    VIGOR 


365 


of  different  species  in  animals  and  plants  do  not  develop  normally. 
Hybrid  weakness  as  well  as  hybrid  vigor  must  be  taken  into  con- 
sideration although  this  is  not  be  to  expected  in  crosses  that  are 
fertile. 

Afier  the  limits  of  physiological  compatibility  are  reached 
cross-fertilization  cannot  be  accomplished.  A  series  can  therefore 
be  arranged  as  follows:  (1)  Crosses  between  organisms  which  are 
so  nearly  alike  in  germinal  constitution  that  no  increased  growth 


.<^>N. 


/  .^^i^i^-^ 


Figure  24.      Crossed  corn  showing  vigorous  growth. 


results.  (2)  Crosses  between  germinally  diverse  but  closely  related 
organisms  that  grow  to  a  larger  size  and  at  a  more  rapid  rate  and 
are  fully  fertile.  (3)  Sterile  crosses  between  more  distantly 
related  organisms  which  are  extremely  vigorous.  (4)  Sterile  crosses 
which  are  weak  and  often  abnormal.  (5)  Crosses  which  cannot 
be  made  on  account  of  the  germinal  difference  in  the  forms  united. 
H^^brid  vigor  in  domestic  animals  and  cultivated  plants  most 
frequently  results  when  breeds  or  varieties  of  different  type  are 
brought  together.     Thus  it  is  a  common  practice  to  cross  the 


366 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


bacon  and  lard  types  of  hogs  or  the  mutton  and  wool  breeds  of 
sheep  to  secure  some  of  the  advantages  of  both  parental  races. 
Dent  and  flint  varieties  of  com  when  crossed  usually  give  greater 
increases  in  yield  than  crosses  within  either  type.  In  these  diverse 
crosses  many  of  the  desirable  features  of  both  parental  races  are 
brought  together.  How  this  works  is  well  illustrated  in  the  cross 
of  a  "golden"  type  of  com  which  is  deficient  in  chlorophyll  with 
a  "dwarf"  as  shown  in  figure  26.  The  plants  resulting  from  this 
cross  are  tall,  normally  green  and  quite  vigorous  and  productive. 
In  this  particular  case  one  parent  contributes  normal  stature  and 
the  other  normal  chlorophyll.  Both  these  characters  are 
dominant  over  the  recessive  condition  so  that  all  the  hybrid  plants 


Figure  25.  It  is  the  uniform  production  of  a  good  ear  on  every  plan- 
that  makes  the  first  generation  hybrids  between  inbred  strains  so  product 
five. 


are  alike  in  their  tall  stature  and  green  color.  Another  case  is 
shown  in  figure  27  of  two  dwarfs  which  are  genetically  different 
and  which,  when  crossed,  give  a  tall,  vigorous  hybrid.  One  of  the 
dwarfs  lacks  something  essential  to  normal  height  and  all  the 
plants  are  alike  as  long  as  they  are  not  out-crossed.  The  other 
dwarf  is  lacking  in  some  other  essential  factor  present  in  normal 
When  these  two  small  plants  are   combined   each  type 


com. 


supplies  what  the  other  lacks  so  that  the  result  is  normal  stature 
in  all  the  hybrid  plants  the  first  year  after  crossing.  These  illus- 
trations of  the  result  of  crossing  are  extreme  cases  which  show  how 
conspicuous  abnormalities  are  suppressed  by  crossing  so  that  the 
hybrid  offspring  are  able  to  make  a  greater  growth  than  either 
parent.    The  same  situation  in  principle  exists  in  all  crosses  from 


AN    INTERPRETATION    OF    HYBRID    VIGOR 


367 


which  hybrid  vigor  ensues.  Different  organisms  possess  different 
hereditary  quahties.  When  brought  together  there  is  always  a 
tendency  for  the  hereditary  factors  which  make  for  greater  growth 
vigor  to  dominate  the  factors  for  lesser  growth.     The  bringing 


Figure  26.  The  result  of  crossing  a  golden,  liguleless  type,  on  the  left, 
with  a  green  dwarf  on  the  right.  The  hybrid,  in  the  center,  has  tall 
stature,  normal  foliage  and  green  chloropliyll  due  to  dominant  factors 
contributed  by  each  parent. 

together  of  the  best  of  both  parents  in  this  way  gives  the  hybrid 
offspring  a  temporarv^  advantage  over  either  parent  in  the  first 
generation  following  the  cross.     Recessive  weaknesses  are  con- 


368 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


tinually  occurrinj^  as  mutations  as  shown  by  the  many  controlled 
observations  on  the  fruit  fly  and  other  forms  of  life.  In  cross- 
fertihzed  organisms,  and  particularly  in  domesticated  animals^ 
and  plants,  crossing  keeps  these  covered  over  and  out  of  sight  by 
combining  them  with  normal  factors.  Many  of  these  recessive 
weaknesses  are   not   distinct  and  visible   characters  as  are   the 


Figure  27.  Two  genetically  different  dwarf  types  give  tall  plants  when 
crossed,  due  to  the  fact  that  the  normal  growth  factor  which  each  lacks 
is  supplied  by  the  other. 

chlorophyll  deficiency  or  dwarfness  in  com  but  nevertheless  they 
weaken  the  organism  in  some  way.  When  such  crossbred  races 
are  inbred,  the  heterozygous  combinations  are  reduced  and  the 
resulting   individuals   which   are   homozygous   to   a   greater   and 


THE    TRANSITORY    NATURE    OF    HYBRID    VIGOR  369 

greater  degree,  as  the  inbreeding  is  continued,  show  the  recessive 
weaknesses  and  are  either  unable  to  reproduce  themselves  or  are 
reduced  in  size  and  rate  of  growth  to  a  point  below  that  of  the 
original  stock.  The  inbred  individuals  each  receive  some  of  the 
hereditary  factors  for  vigorous  growth.  Some  receive  more  than 
others  as  a  chance  allotment  and  are  therefore  better  able  to  sur- 
vive the  inbreeding  process.  Others  are  so  weakened  that  they 
perish.  On  account  of  the  way  in  which  the  hereditary  mechanism 
operates  it  is  extremely  improbable  that  any  one  individual  will 
receive  all  the  more  favorable  growth  factors,  and  in  actual  practice 
inbred  strains  of  com  are  all  reduced  by  inbreeding.  It  is  theo- 
retically possible  to  obtain  individuals  which  possess  an  unusually 
large  share  of  the  more  favorable  growth  factors  or  even  all  of  them 
and  for  that  reason  show  no  reduction  from  inbreeding.  Darwin 
obtained  self-fertilized  races  of  Iponiea  and  Mimulus  which  were 
more  vigorous  than  the  naturally  cross-fertiHzed  variety  at  the 
start.  Cummings  reports  self -fertilized  strains  of  squash  that  are 
as  productive  as  the  original  variety  and  much  more  uniform  in 
type.  King  has  obtained  inbred  rats  after  long-continued  brother 
and  sister  mating  that  are  fully  as  vigorous  as  the  material  with 
which  she  started.  The  fact  that  no  such  result  has  been  ob- 
tained with  com  shows  how  dependent  this  plant  has  become 
upon  cross-fertilization  to  maintain  production. 

The  Transitory  Nature  of  Hybrid  Vigor. 

The  increased  growi;h  resulting  from  crossing  is  quickly  lost  in 
the  following  generations  when  the  h^-brid  individuals  are  bred 
among  themselves  or  again  inbred.  In  other  words,  hybrid  vigor 
is  a  temporan,'  manifestation  which  ordinarily  cannot  be  fixed  and 
made  permanent  in  sexually  reproduced  offspring.  The  reason  for 
this  is  readily  appreciated  when  the  illustrations  previously  given 
are  followed  into  the  later  generations.  The  cross  of  the  golden 
and  dwarf  com  gives  all  normal  tall  green  plants  in  the  first 
hybrid  generation.  Seed  from  these  h^^brid  plants,  either  selfed 
or  inter-crossed,  always  gives  in  the  next  generation  aU  the  possible 
combinations  of  characters  that  went  into  the  cross.  In  this 
particular  case  the  golden  plants  also  lacked  the  ligule  which  is 
the  small  extension  of  the  leaf  sheath  surrounding  the  stalk  above 
the  leaf  blade.  Liguleless  plants  hold  their  leaves  in  a  characteris- 
tically upright  position  close  to  the  stalk.  In  the  second  generation 
of  this  cross  of  liguleless  golden  by  dwarf,  eight  different  kinds  of 
plants  are  produced.  These  are  shown  in  figure  28.  Due  to  the  re- 
combination of  Mendelian  units,  this  generation  is  extremely 
variable,  and  while  some  of  the  tall,  green,  liguled  plants  may  be  as 
vigorous  and  productive  as  the  first  crossed  plants  this  generation 
as  a  whole  averages  much  less  productive.  By  further  inbreeding, 
eight  distinct  pure-breeding  combinations  of  these  three  characters 


370 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


can  be  obtained  and  within  each  type  still  further  minor  differences 
could  be  established.  Crossing  any  two  of  these  types  gives 
increased  growth  and  restores  the  normal  condition  provided  the 
factors  for  normal  growth  are  all  present  in  one  or  the  other  type. 
In  the  same  way  the  vigorous  and  productive  crosses  between 
inbred  strains  of  com  fall  off  in  size  and  yield  in  the  second  genera- 
tion and  are  much  more  variable.  This  always  results  whether 
the  first  crossed  plants  are  self -fertilized  or  are  inter-crossed  among 
themselves.  If  the  inbred  strains  are  uniform  and  fixed  in  their 
type  the  first  generation  hybrid  plants  are  germinally  all  alike  so 


Figure  28.  The  second  generation  offspring  from  the  crossing  of  golden 
Uguleless  by  dwarf.  Eight  different  combinations  of  these  three  characters 
are  obtained  by  Mendehan  segregation  and  recombination. 


that  it  is  easily  understood  why  self-fertilization  and  inter-crossing 
give  the  same  result.  To  test  this  out  two  inbred  strains  were 
crossed  after  14  generations  of  self-fertilization.  A  number  of  the 
hybrid  plants  were  self -fertilized  and  an  equal  number  were  inter- 
pollinated.  The  seed  of  these  two  lots  was  planted  in  alternate 
rows,  replicated  three  times.  The  self -fertilized  plants  averaged 
76. 2 ±.57  inches  in  height  in  comparison  with  the  intercrossed 
plants  which  averaged  73. 8 ±.70.  In  production  of  grain  they 
stood  respectively  22. 2 ±1.2  and  22.0 ±2.4  bushels  per  acre.  In 
neither  case  are  the  differences  significant. 


inbreeding  after  crossing  371 

Inbreeding  after  Crossing. 

When  the  second  generation  plants  are  allowed  to  intercross 
naturally  no  further  reduction  in  vigor  is  expected.  Variability 
and  yield  should  remain  at  the  same  level  thereafter  until 
natural  or  artificial  selection  eliminates  certain  strains.  But  when 
the  second  generation  plants  are  self -fertilized  there  is  a  further 
reduction  in  size,  and  if  the  inbreeding  is  continued  the  decline  in 
size  and  vigor  and  in  variability  proceeds  in  approximately  the 
same  way  as  when  the  parental  strains  were  first  inbred.  This  is 
shown  in  figures  29,  30  and  31. 

In  this  demonstration  of  inbreeding  after  crossing,  two  inbred 
strains,  self-fertilized  for  eight  generations,  were  crossed  and  the 
first  generation  plants  again  self -fertilized.  In  the  second  genera- 
tion a  single  plant  was  again  chosen  as  the  progenitor  and  polli- 
nated in  the  same  way,  and  this  was  continued  for  eight  successive 


Figure  29.  The  result  of  inbreeding  after  crossing.  Two  inbred  strains 
at  the  left,  their  first  generation  hybrid  adjoining,  followed  by  seven  suc- 
cessive generations  self-fertilized. 

generations.  Seed  was  saved  from  each  year's  selfing  up  to  the  fifth 
generation.  Since  com  seed  will  not  retain  its  germination  satis- 
factorily for  more  than  six  years,  single  plants  were  again  self- 
fertilized  the  fifth  year  in  each  generation  and  this  seed  was  used 
from  then  on.  All  eight  inbred  generations  were  growm  in  1923 
along  with  the  two  parental  strains  as  shounti  in  the  accompanying 
illustrations.  This  demonstration  has  been  gro\\Ti  each  year 
since  the  original  cross  was  made  and  the  yields  obtained  in  the 
different  years  are  given  in  table  VII.  Production  has  varied 
rather  widely  from  season  to  season  and  from  generation  to  genera- 
tion. This  is  due  in  part  to  the  character  of  the  individual  plants 
chosen  for  progenitors.  A  ver}^  noticeable  drop  takes  place  from 
the  first  to  the  second  generation  amotuiting  to  over  30  per  cent, 
as  an  average  of  the  six  years.  Kiesselbach  tested  the  first  and 
second  generations  of  eight  hybrid  combinations  of  different  strains 
during  two  seasons  and  obtained  an  average  of  52.2  and  27.8 
bushels  per  acre  respectively  for  the  two  generations,  to  be  com- 


372  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 

pared  with  41.7  bushels  for  the  original  com  from  which  the 
inbred  strains  were  obtained.  He  secured  his  seed  for  the  second 
generation  by  pollinating  several  first  generation  plants  with 
composite  pollen  from  15  sib  plants.  The  reduction  from  the  first 
to  the  second  generation  of  nearly  50  per  cent,  is  even  greater  than 
in  our  case  where  the  plants  were  self -fertilized.  Kiesselbach  also 
grew  a  third  generation  from  seed  of  interpollinated  plants.  The 
comparative  yields  obtained  for  the  first,  second  and  third  genera- 
tions were  51.5,  29.4,  and  25.6  bushels  per  acre.  The  reduction 
from  the  second  to  the  third  as  would  be  expected  from  this  mode 
of  pollination  is  small  compared  with  the  drop  from  the  first  to 
the  second.  Continued  inter-pollination  should  cause  no  further 
decrease  in  yield  unless  particularly  unfavorable  strains  are 
isolated. 

The  average  height  of  these  successive  self -fertilized  genera- 
tions compared  with  the  first  generation  hybrid  and  the  parental 
strains  is  shown  graphically  in  figure  32.     There  is  a  continued 

Table  VII. 

The  production  of  grain  in  bushels  per  acre,  of  two  inbred  strains  of 
corn  and  their  hybrid  and  the  Fi  to  the  Fs  generations  successively  self- 
fertilized. 


Year 

Generati 

ons 

Grown 

Pa 

Pb 

Fi 

F2 

F, 

F4 

Fi 

Fs 

F7 

Fs 

1917 

22 

6 

65 

56 

1918 

27 

24 

121 

128 

is' 

1920 

16 

28 

128 

48 

35 

'29' 

io' 

1921 

20 

13- 

73 

55 

49 

33 

15 

'23" 

1922 

20 

26 

160 

83 

74 

68 

49 

36 

'2.3' 

1923 

13 

21 

61 

45 

41 

47 

16 

23 

26 

27" 

Ave. 

20 

20 

101 

69 

43 

44 

23 

27 

25 

27 

reduction  in  each  generation,  but  the  decrease  is  much  less  during 
the  last  three  generations  than  in  the  first  four.  From  the  first  to 
the  fifth  generation  there  is  a  decline  of  27.2  inches  in  stature  and 
from  the  fifth  to  the  eighth  8.6  inches.  The  rate  of  growth  as 
measured  by  the  daily  gain  in  height  is  also  steadily  reduced  as 
shown  in  figure  33,  the  decline  being  greater  during  the  first  stage 
of  inbreeding  than  in  the  last.  The  dift'erences  between  the  last 
two  generations  in  all  measurable  characters,  including  yield, 
height,  length  of  ear  and  rate  of  growth,  are  so  small  that  it  seems 
evident  that  the  reduction  in  size  and  vigor  is  rapidly  approaching 
an  end.  The  last  two  generations  are  so  similar  in  appearance 
that  they  cannot  be  distinguished  in  the  field.  In  tassel  type, 
foliage  character,  position  of  the  ear  on  the  stalk,  and  in  the  size 
and  conformation  of  the  ears  these  two  generations  are  practically 
identical. 

The  reduction  in  variability  from  the  first  to  the  eighth  genera- 
tion was  very  noticeable  in  the  field.  One  of  the  parent  strains 
has  green  silks,  the  other  red.     The  first  generation  hybrid  plants 


INBREEDING    AFTER    CROSSING 


373 


all  had  red  silks.  The  second,  third  and  fourth  generations  segre- 
gated for  this  color  while  the  remaining  generations  were  all 
uniformly  colored.  Height  of  plant,  position  of  the  ear  on  the 
stalk,  form  of  tassel  and  all  structural  details  were  noticeably  uni- 
form in  the  parents  and  the  first  hybrid  generation.  The  plants 
in  the  generations  from  the  second  to  the  fifth  were  quite 
variable  but  later  became  more  and  more  uniform  until  in  the 
last  two  generations  they  showed  as  little  variation  as  either  of 
the  parental  strains. 

The  inbred  strain  which  resulted  from  this  second  period  of  self- 
fertilization  differs  from  both  parental  strains.  In  tassel,  ear,  and 
character  of  the  foliage  it  is  quite  unlike  either  but  is  noticeably 
susceptible  to  smut   like  one  of  the  parents.      In  other  words, 


Figure  30.      Inbreeding  after  crossing 
generations  shown  in  figure  29. 


Representative  plants  from  the 


Mendelian  recombination  has  taken  place  so  that  the  details  of 
structure  are  altered.  Apparently  this  inbred  strain  has  about 
the  same  nimiber  of  favorable  growth  factors,  and  for  that  reason 
it  is  no  better  or  no  worse  than  the  parental  stocks  that  went  into 
the  vigorous  and  productive  hybrid  from  which  the  new  strain  was 
derived  a' few  generations  before. 

For  all  practical  purposes  the  reducing  effect  of  self-fertilization 
in  this  particular  case  has  ceased  at  the  sixth  inbred  generation. 
This  closely  parallels  the  course  of  events  when  the  parental 
strains  were  first  inbred.  Theoretically  the  loss  of  vigor  follows 
the  rule  of  halving  the  remaining  dift'erence  in  each  generation. 
If  we  take  an  individual  heterozygous  for  a  single  Mendelian  pair 
of  factors  such  as  Aa  we  expect  in  the  next  generation  fifty  per 


374  CONNECTICUT    EXPERIMENT    STATION  BULLETIN   266. 

cent,  of  the  plants  homozygous  for  this  pair  of  factors  and 
having  the  composition  A  A  or  aa;  the  other  fifty  per  cent,  will 
on  the  average  still  be  heterozygous  for  this  factor  pair;  i.  e.  Aa 
in  composition.  In  choosing  a  single  self -fertilized  individual  for 
the  progenitor  the  chances  are  even  that  it  will  be  homozygous  or 
heterozygous.  This  holds  for  any  number  of  factor  pairs  and 
since  each  pair  when  once  alike  must  remain  so  thereafter  in  self- 
fertilization  the  niunber  of  mixed  pairs  is  steadily  reduced  by 
half  in  each  generation.  Starting  with  an  individual  100  per  cent, 
heterozygous,  the  following  generations  would  be  on  the  average 
50,  25,  12.5,  6.25,  3.125,  1.5625,  etc. 

Naturally  the  progeny  of  any  heterozygous  individual  will  vary 
greatly  in  composition.  Some  will  be  nearly  or  completely  homo- 
zygous while  others  will  be  nearly  or  completely  heterozygous 
with  respect  to  all  factor  pairs.  For  that  reason  the  result  of  any 
process  of  inbreeding  depends  entirely  upon  the  composition  of  the 
individual  plants  which  are  chosen  as  progenitors.  It  is  theoreti- 
cally possible  to  obtain  individuals  in  each  generation  which  are 
as  heterozygous  as  their  parents  and  others  that  are  completely 
homozygous.  For  that  reason  inbreeding  may  cause  no  reduction 
in  size,  vigor  or  variability,  or  complete  reduction  may  take  place 
in  a  single  generation.  The  chances  that  such  a  result  will  be 
obtained,  however,  are  extremely  remote.  Actually  the  reduction 
follows  the  rule  of  halving  the  remaining  difference  very  closely 
so  that  it  is  evident  that  a  very  large  number  of  factors  play  a 
part  in  hybrid  vigor.  How  many  such  factors  there  are,  we  have 
no  way  of  estimating  at  the  present.  Many  factors  which  bring 
about  visible  differences  possibly  have  no  effect  upon  vigor  but 
apparently  the  number  of  them  which  are  essential  to  normal 
development  in  com  is  exceedingly  great. 

The  Attainment  of  Complete  Homozygosity. 

Whether  complete  fixity  of  type,  absolute  homozygosity,  is 
possible  of  attainment  by  continuous  self-fertilization  has  been 
•previously  discussed.  (Jones  1924.)  The  experimental  results 
show  that  small  germinal  differences  may  remain  after  many 
generations  of  inbreeding.  Two  lines  separated  from  one  in  the 
third  generation  and  then  continued  separately  for  several  genera- 
tions gave  a  marked  increase  in  size  when  crossed,  although  not  as' 
great  as  in  the  case  of  lines  separated  at  the  beginning,  showing 
that  two  self-fertilizations  had  not  produced  much  uniformity  in 
germinal  constitution.  The  four  original  Learning  strains  were 
continued  as  single  lines  up  to  the  eighth  generation.  At  that 
time  they  were  all  remarkably  uniform  and  apparently  fixed  in 
their  type.  Then  each  line  was  separated  into  two  lines  which 
were  continued  separately  thereafter  for  eight  or  more  additional 
generations.  At  that  time  two  of  the  paired  lines  had  remained 
exactly  alike.     No  visible  differences  in  any  respect  could  be  seen. 


MUTATIONS    IN    CORN  375 

One  of  the  paired  lines  differed  only  in  color  of  the  seeds,  one  being 
noticeably  brighter  in  color  in  some  seasons.  As  the  growing 
conditions  were  alike  for  all  plants  this  slight  difference  can  not  be 
accounted  for  in  any  other  way  than  as  an  heritable  difference. 
The  other  paired  line  differed  noticeably  in  many  respects.  One 
of  the  members  was  taller,  the  leaves  were  broader  and  lighter 
colored  and  the  ears  were  larger,  the  seeds  broader  and  duller 
in  color. 

Crossing  these  paired  lines  gave  significant  increases  in  all 
measurable  characters  in  the  one  strain  whose  paired  lines  were 
visibly  different.  The  other  strains  all  showed  slight  but  appar- 
ently significant  increases  in  some  characters.  The  two  strains 
whose  paired  lines  showed  no  visible  differences  were  again  tested 
after  fourteen  generations  of  self-fertilization  in  the  following  way. 
The  two  strains  which  were  distinct  from  the  beginning  were  cross- 
ed and  gave  the  usual  vigorous  and  uniform  hybrid  plants.     A 


Figure  31.      Inbreeding  after  crossing.      The  production  of  grain  from 
the  plants  shown  in  figure  29. 

number  of  these  were  self-fertilized  and  an  equal  number  were 
inter-pollinated  by  sib  plants.  A  careful  test  failed  to  show  any 
differences  in  size  or  productiveness  in  the  plants  grown  from  these 
two  lots  of  seed.  If  the  parental  strains  were  not  germinally  alike 
within  themselves,  intercrossing  the  first  generation  hybrid  plants 
would  not  cause  such  a  decrease  in  heterozygosity  as  self-fertiliza- 
tion. The  fact  that  no  diff'erence  was  shown  indicates  that  the 
parental  strains  were  completely  homozygous  for  all  factors  which 
influence  gro^vth  vigor.  However,  this  test  is  not  a  ver\'  delicate 
one  and  final  proof  awaits  the  crossing  of  the  paired  lines  which 
have  been  separated  in  the  seventeenth  generation  and  will  be 
carried  along  for  several  additional  generations. 

Mutations  in  Corn. 

Complete  homozygosity  may  be  impossible  to  attain  because  of 
spontaneous  variations,  mutations,  occurring  from  time  to  time. 


376 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


During  the  seventeen  years  in  which  the  four  inbred  Learning 
strains  have  been  under  observation  only  two  apparent  germinal 
changes  have  been  recorded.  Until  a  fairly  high  degree  of  uniform- 
ity was  reached,  after  six  generations,  various  abnormalities 
occurred  singly  or  in  greater  numbers  in  the  rather  small  progenies 
that  were  grown.  Presumably  these  were,  at  least  in  the  great 
majority  of  cases,  merely  segregations  from  a  heterozygous  com- 
plex. But  new  characters  appearing  after  uniformity  is  obtained 
which  have  not  been  noted  previously  have  every  indication  of 
being  mutations.  Two  such  have  been  observed  in  different  lines. 
One  produced  in  the  thirteenth  generation  a  single  self -pollinated 
ear  segregating  for  defective  seeds.  All  of  the  lines  had  been 
examined  for  the  new  character  during  three  previous  generations, 
without  noting  anj^thing  of  this  kind,  and  since  the  character 


Figure  32.      Graph  showing  the  height  of  the  two  parental  strains    and 
the  generations  from  the  Fi  to  Fs. 

segregated  as  a  single  Mendelian  recessive  when  out-crossed,  there 
is  every  reason  to  assimie  that  a  germinal  change  took  place 
shortly  before  its  appearance.  Among  approximately  a  thousand 
plants  of  another  line,  self -fertilized  more  than  ten  generations, 
which  has  always  produced  white  cobs,  four  ears  were  found  with 
light  red  cobs.  The  cobs  of  this  strain  are  flattened  and  the 
plants  are  otherwise  easily  identified.  The  red  cob  plants  were 
examined  at  harvest  and  noted  to  be  typical  for  the  strain  in  all 
respects  except  cob  color.  Neither  of  these  changes  could  have 
been  due  to  out-crossing.  Stray  pollen  from  any  outside  source 
immediately  results  in  vigorous  plants  twice  as  large  as  the  inbred 
plants  ever  grow  and  the  crossed  plants  are  completely  changed  in 
type.  Since  the  mutant  plants  were  in  other  respects  typical 
plants  of  the  strain  and  were  no  larger  they  could  not  have  resulted 
from  out-crossing. 

Two  additional  changes  have  occurred  in  other  inbred  material 


THE    VALUE    OF    INBREEDING 


377 


such  that  they  have  every  indication  of  being  recent  germinal 
alterations.  One  strain  after  five  generations  produced  for  the 
first  time  striped,  variegated  plants  which  bore  no  pollen  or  seed. 
They  occured  in  later  generations  in  about  25  per  cent,  of  the  off- 
spring from  normal  plants.  Another  strain  after  nine  generations 
gave  small  narrow-leaved  dwarf  plants  which  were  quite  distinct 
from  the  normal  plants.  They  produced  a  small  amount  of  pollen 
and  when  out-crossed  to  normal  plants  they  reappeared  in  later 
generations  showing  that  the  change  was  heritable. 

These  four  apparent  mutations  are  all  that  have  been  noted  in  a 
large  number  of  uniform  strains  which  have  been  under  obsen-ation 
for  many  years.  Hayes  and  Brewbaker  record  the  production  of 
chlorophyll  deficient  seedlings  in  four  lines  out  of  953  which  had 


Figure  33.      Graphs   showing   rate   of   growth    (average   daily   gain   in 
height)  for  the  same  generations  as  in  the  preceding  ilhistrations. 

not  shown  such  abnormalities  previousl^^  In  these  cases  the 
appearance  of  the  abnormalities  may  have  been  due  to  delayed 
segregation,  since  the  lines  had  not  been  reduced  to  uniformity  and 
constancy.  While  it  is  evident  that  com  does  mutate,  the  fre- 
quency of  these  changes  is  so  low  that  inbred  strains,  when  once 
reduced  to  uniformity,  are  stable  for  all  practical  purposes.  Some 
care  will  be  needed  to  maintain  self -fertilized  lines  true  to  type,  and 
when  recessive  abnormalities  appear  those  progenies  which  show 
them  will  have  to  be  discarded. 


The  Value  of  Inbreeding. 

This  review  of  the  effects  of  inbreeding  and  crossing  upon  com 
has  been  given  in  considerable  detail  because  the  facts  learned  from 


378 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


these  investigations  form  the  basis  for  the  method  of  ini])rovement 
by  selection  in  self-fertihzed  Hnes.  In  the  inbreeding  experiments 
just  described  no  selection  of  superior  individuals  to  perpetuate  the 
strain  was  made.  The  aim  was  to  take  normal  plants  at  random 
and  note  the  outcome.  Nevertheless  a  great  deal  of  natural 
selection  has  taken  place.  All  abnormalities  which  interfere  with 
or  markedly  reduce  reproductive  ability  have  been  automatically 
eliminated.  In  this  way  many  chlorophyll  deficiencies,  endosperm 
abnormalities  and  inherited  sterility  in  tassels  and  ears,  unfavor- 
able conditions  almost  always  present  in  every  cross-pollinated 


75 


Figure  34.  A  diagrammatic  representation  of  the  actual  and  theoretical 
results  of  inbreeding  corn.  The  solid  lines  represent  strains  which  have 
already  been  obtained,  the  dotted  lines  those  which  may  be  expected  when 
corn  is  worked  with  more  extensively. 


variety  of  corn,  have  been  cleaned  out.  But  this  outcome  of  in- 
breeding, valuable  as  it  may  be,  is  less  important  than  the  control 
over  the  heredity  made  possible  by  hand  pollination  and  the  result- 
ing fixity  of  type. 

In  common  practice,  selection  with  nearly  all  cross-fertilized 
plants  has  been  based  on  the  appearances  of  the  plant  or  upon  the 
performance  of  the  progeny,  and  no  adequate  control  of  the 
heredity  brought  in  from  the  pollen  parent  has  been  possible.  As 
generally  practised,  corn  breeding  has  been  similar  to  a  system  of 
animal  breeding  in  which  selection  is  carried  on  only  with  the 
dams  paying  no  attention  whatever  to  the  sires.     The  disastrous 


THE    VALUE    OF    INBREEDING 


379 


result  that  such  a  system  would  have  upon  purebred  live-stock 
can  readily  be  appreciated.  With  all  cross-fertilized  plants  it 
would  be  theoretically  possible  to  follow  the  method  now  used  in 
animal  breeding.  Certain  desirable  individuals  could  be  chosen 
as  seed  parents  and  others  as  pollen  parents.  Pollination  could  be 
made  by  hand  and  the  progenies  compared  on  the  basis  of  their 
performance.  There  is  no  doubt  that  this  system  followed  up  as 
carefully  as  it  is  in  mating  farm  animals  would  give  equal  results. 
But  such  a  method  is  wholly  impracticable  on  account  of  the  small 
value  of  the  individual  plant.     The  time  spent  on  selecting  the 


Figure  35.  Self-pollinated  ears  grown  on  selected  plants  of  Burweirs 
Yellow  Flint,  No.  40.  Each  ear  is  the  starting  point  of  a  selected  Hne. 
These  are  numbered  1  to  9,  top  row,  and  10  to  IS,  bottom  row,  left  to  right. 

parents  and  on  polHnating  each  generation  would  not  be  repaid 
by  the  possible  gains.  Furthermore,  with  com,  selection  is  greatly 
handicapped  due  to  the  fact  that  the  principal  objective,  pro- 
duction of  grain,  is  not  visible  until  after  pollination. 

A  new  method  of  attack,  which  will  make  possible  a  control  of 
the  heredity  transmitted  thru  the  pollen  as  well  as  thru  the  egg,  is 
needed  for  all  naturally  cross-fertilized  plants.  Since  inbreeding 
is  a  sorting-out  process,  selection  carried  on  dtrring  the  time  the 
plants  are  being  reduced  to   uniformitv  and  constancv  makes 


380  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 

it  possible  to  look  for  desirable  qualities  with  a  certainty  of  being 
able  to  hold  them,  when  once  secured,  that  has  never  before  been 
possible.  From  this  viewpoint  inbreeding  is  not  so  important  as  a 
method  of  gaining  the  maximum  effect  of  hybrid  vigor  when  the 
inbred  strains  are  crossed  as  it  is  of  separating  out  and  making 
visible  the  very  best  hereditary  qualities  that  may  exist  in  a 
heterozygous  stock.  Strains  when  once  reduced  to  fixity  remain 
the  same  indefinitely,  barring  mutations.  With  due  regard  to 
seasonal  variation,  crosses  between  inbred  strains  give  the  same 
result  whenever  the  same  combination  is  made.  The  uniform 
production  of  the  first  generation  hybrids  between  homozygous 
strains  is  an  important  feature.  In  this  respect  cross-fertilized 
plants  are  equal  to  self -fertilized  plants  in  uniformity  and  fixity  of 
type  and  have  the  added  advantage  of  crossing  to  bring  together 
and  use  in  the  first  generation  the  desirable  qualities  within  the 
species,  which  in  a  self -fertilized  organism  can  be  used  only  when 
recombined  and  fixed  in  a  homozygous  condition.  It  should  there- 
fore be  clearly  understood  that  the  crossing  of  inbred  strains  as 
such  is  without  particular  value  and  that  the  opportunity  afforded 
to  find  and  to  fix  the  very  best  hereditary  qualities  possessed  by  a 
cross-bred  race  is  the  more  important  function  of  inbreeding. 
Crossing  is  merely  a  means  of  utilizing  this  good  heredity  by  giving 
it  maximum  vigor.  It  is  to  be  expected  that  many  inbred  strains 
will  have  only  medium  value  and  give  no  improvement  over  the 
original  variety  when  crossed.  The  bulk  of  the  germplasm  in 
every  population  is  mediocre.  Of  necessity  only  the  exceptionally 
few  will  give  outstanding  results.  For  these  reasons  the  outcome 
of  selection  in  self-fertilized  lines  depends  upon  how  extensively 
and  skillfully  it  is  applied. 

Possibility  of  Obtaining  Vigorous  Inbred  Strains. 

Most  of  the  inbred  strains  of  corn  so  far  produced  have  been 
reduced  to  about  fifty  per  cent,  or  less  of  the  production  of  the 
original  cross-bred  varieties.  Some  strains  have  failed  to  repro- 
duce after  one  generation  of  self-fertilization.  Others  have  per- 
sisted in  a  weakened  condition  for  several  generations  and  then 
perished.  Still  other  strains  are  able  to  survive,  but  are  continued 
only  with  the  greatest  difficulty.  The  majority  of  the  self- 
fertilized  lines,  when  uniformity  and  fixity  of  type  are  reached,  are 
about  one-third  as  productive  as  at  the  start.  A  few  are  exception- 
ally good.  They  grow  more  vigorously  and  yield  more  than  the 
rest  and  are  equally  uniform  and  fixed  in  their  type.  But  even  the 
best  of  these  are  still  below  the  original  variety  in  amount  or 
quality  of  grain  produced.  On  the  basis  of  hybrid  vigor  being 
due  to  dominance  of  the  more  favorable  factors  it  is  theoretically 
possible  to  secure  inbred  strains  that  will  show  little  or  no  reduc- 
tion in  vigor,  and  a  few  may  sometime  be  obtained  that  are  even 


OBTAINING    VIGOROUS    INBRED    STRAINS 


381 


m(3re  vigorous  and  productive  than  the  cross-bred  variety.  This 
is  deduced  from  the  fact  that  most  heterozygous  combinations  of 
factors  are  less  effective  than  the  homozygous  combinations  of  the 
same  factors.  Thus  the  cross  of  yellow  and  white  corn  gives  a 
lighter  color  than  pure  yellow.  The  cross  between  a  determinate 
gro\\i;h  type  of  tobacco  with  an  indeterminate  growth  type  (Jones, 
1921)  which  involves  a  single  factor,  differs  from  either  parent  in 
size  of  plant  and  number  of  leaves.  Dominance  is  seldom  perfect 
and  while  there  is  little  direct  evidence  in  this  respect  for  characters 


Figure  .36.  Self-pollinated  ears  grown  on  selected  plants  of  Gold 
Nugget,  No.  105.  Each  ear  is  the  starting  point  of  a  selected  line.  These 
are  numbered  2  to  10,  top  row,  and  11  to  20  bottom  row,  left  to  right. 
(Ear  1  was  shelled  before  photographing.     It  was  similar  to  No.  2.) 


which  directly  affect  vigor  there  is  every  reason  to  expect  that  a 
homoz^'gous  combination  of  all  the  more  favorable  dominant 
growth  factors  will  make  possible  a  greater  development  than  the 
heterozygous  combinations  of  the  same  factors  with  weaker  allelo- 
morphs. However,  as  just  noted,  certain  results  are  obtained  from 
heterozygous  combinations  that  can  not  be  obtained  from  either 
factor  alone.  If  there  are  many  of  these  that  play  a  part  in  growth 
vigor,  then  heterozygosity  may  be  indispensable  to  maximum 
development.     Moreover,    recombinations    of    large    nimiber    of 


382 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


factors  are  extremely  difficult  to  obtain  and  since  favorable  and 
unfavorable  growth  factors  are  distributed  indiscriminately 
throughout  the  hereditary  mechanism  the  chances  of  securing 
self-fertilized  strains  of  com  which  equal  the  cross-bred  varieties 
are  so  exceedingly  small  that  there  is  little  hope  of  obtaining  them. 
The  most  that  can  reasonable  be  expected  are  inbred  strains  which 
are  appreciably  better  than  any  that  have  so  far  been  produced. 
The  results  that  have  already  been  obtained  from  self-fertilizing 
corn,  and  the  theoretical  possibilities,  some  of  which  may  be  attain- 
ed in  the  future,  are  shown  diagrammatically  in  figure  34. 


Figure  37.  Self-pollinated  ears  grown  on  selected  plants  of  Century 
Dent,  No.  110.  Each  ear  is  the  starting  point  of  a  selected  line.  These 
are  numbered   1  to  9  top  row  and   10  to   18, bottom  row,  left  to  right. 

Selection  in  Self-Fertilized  Lines. 

To  demonstrate  the  value  of  inbreeding  as  a  means  of  isolating 
good  heredity  a  system  of  selection  in  self -fertilized  lines  was  begun 
in  1918.  Four  varieties  of  com  were  chosen  as  material  with  which 
to  work.  These  varieties  have  been  grown  in  Connecticut  for 
many  years  and  are  well  adapted.  In  a  variety  test  of  long 
duration  they  have  proven  to  be  among  the  best  in  production  of 
grain  and  in  other  qualities.     The  four  varieties  are  as  follows : 

Burwell's  Yellow  Fhnt,  No.  30  and  No.  40.  An  eight  rowed 
yellow  com  of  the  Canada  Flint  type.  The  ears  are  medium  in 
size,  one  or  two  on  the  stalk.     The  plants  are  medium  in  maturity. 


SELECTION    IN    SELF-FERTILIZED    LINES 


383 


Gold  Nugget,  No.  105.  An  eight  rowed  yellow  flint  com  with 
large  ears,  broad  kernels  and  heavy  cobs.  The  stalks  are  large 
with  few  suckers.     The  plants  mature  late  in  the  season. 

Century  Dent,  No.  110.  A  light  yellow  dent  corn  with  broad, 
smooth,  shallow  dented  kernels.  The  ears  are  medium  in  size 
and  have  from  14  to  18  rows.  The  plants  are  medium  in  size  and 
mature  well  in  practically  every  season. 

Beardsley's  Learning,  No.  112.  A  yellow  dent  corn  with  taper- 
ing ears  with  16  to  22  rows  and  small,  shallow  kernels.  The  stalks 
are  large.  This  variety  is  later  in  maturing  than  Century  Dent 
and  is  usually  more  productive. 


Figure  38.  Self-pollinated  ears  grown  on  selected  plants  of  Beardsley's 
Learning,  No.  112.  Each  ear  is  the  starting  point  of  a  selected  line.  These 
are  nunAered  1  to  8,  top  row,  and  9  to  16  bottom  row. 


The  plan  of  procedure  was  to  self -fertilize  a  niunber  of  the  best 
plants  in  each  of  these  four  varieties  and  to  use  each  of  these  plants 
as  the  starting  point  of  an  inbred  line.  These  lines  were  to  be 
continued  by  self-pollination  of  the  best  plants  in  each  generation 
until  uniformity  and  constancy  were  reached.  Accordingly  from 
about  60  plants  each  of  the  four  ^^arieties  gro\\Ti  from  a  general 
mixed  lot  of  seed,  20  plants  of  each  variety  were  selected  at 
pollinating  time  and  self-fertilized.  These  four  lots  of  ears  are 
shown  in  figures  35  to  38.     Some  of  the  seh'-pollinated  plants 


384 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


failed  to  set  seed  but  all  of  the  ears  that  had  enough  seed  to  work 
with  were  planted.  The  original  hand-pollinated  ears  were  ranked 
according  to  their  appearance  in  size,  form  of  the  ear  and  quality 
of  the  seed.  Ear  number  one  represents  the  best,  number  two 
the  next  best  and  so  on  down.  The  ear  numbers  became  the 
numbers  for  the  self -fertilized  lines  derived  from  them.  Therefore, 
the  number  of  the  line  shows  how  its  original  progenitor  was  classi- 
fied. It  is  of  considerable  interest  to  note  to  what  extent  good 
strains  can  be  obtained  from  unpromising  ears  at  the  start. 

Each  self -pollinated  ear  was  planted  in  a  row  the  following  year 
and  five  plants  of  each  were  again  selected  at  pollinating  time  as 
the  most  desirable  and  were  self -fertilized.  It  was  noted  that  the 
best  appearing  plants  at  the  tirrie  of  pollination  were  not  always 


••PLANTS  oj   : 
ORIGINAL  VARIETY 


LINE  A 
I 


LINE  C 
i 


o  o 


Figure  39.  Diagram  of  a  method  of  selection  in  self-fertilized  lines. 
An  individiial  plant  becomes  the  starting  point  of  each  inbred  strain. 
Three  progenies  are  grown  but  only  one  is  selected  to  continue  the  line. 

the  most  productive  at  maturity.  For  this  reason  more  plants 
were  self -pollinated  than  there  were  progenies  planted,  thus  allow- 
ing for  some  failures  of  pollination  and  also  to  permit  of  some 
selection  among  the  hand  pollinated  ears.  Also,  in  order  to  base 
selection  upon  progeny  performance  rather  than  upon  the  appear- 
ance of  the  seed  ear,  three  progenies  from  each  line  were  grown  each 
year.  At  pollinating  time  the  best  appearing  progeny  was  chosen 
and  five  plants  were  again  self-fertilized,  the  other  two  progenies 
being  discarded.  This  method  of  carrying  on  selection  is  shown 
diagrammatically  in  figure  39. 

About  thirty  plants  were  grown  in  each  progeny.  From  three 
to  five  times  this  niimber  of  seeds  was  planted  and  the  poorest 


SELECTION    OF    EARS    FOR    PLANTING  385 

seedlings  pulled  out  after  they  were  well  started,  leaving  the  tallest 
and  most  vigorous  plants.  An  even  stand  was  obtained  in  most 
cases.  The  end  plants  in  each  row  were  usually  avoided  in 
selecting  the  plants  for  hand-pollination  as  these  are  nearly  always 
larger  and  better  developed  than  the  others  on  account  of  their 
better  opportunity  to  grow. 

METHOD    OF    POLLINATION. 

The  plants  were  pollinated  by  hand  as  shown  in  figures  40  and  41 . 
The  general  method  used  is  as  follows:*  A  three  pound  manila 
grocer's  bag  is  placed  over  the  ear  shoot  before  the  silks  appear. 
The  tassels  are  covered  with  an  eight  or  ten  pound  bag  as  soon  as 
they  are  above  the  upper  leaves.  When  the  silks  are  about  three- 
fourths  out,  pollen  is  dusted  over  them  and  the  tassel  bag  placed 
over  the  ear.  Care  is  taken  not  to  touch  the  silks  or  the  inside  of 
the  tassel  bags  with  the  hands  in  order  to  "avoid  contamination 
with  foreign  pollen.  If  the  silks  extend  more  than  three  or  four 
inches  beyond  the  tip  of  the  ear  the}^  are  cut  back  with  a  knife 
sterilized  in  alcohol.  After  the  first  generation  or  two,  out-crossed 
plants  can  be  easily  noted  by  their  much  greater  size  and  darker 
green  color  so  that  contaminating  pollen  is  not  a  cause  for  great 
concern.  Effort  is  made  to  pollinate  as  rapidly  as  possible.  Only 
one  application  of  pollen  is  made.  If  sufficient  seed  does  not  result 
from  this  application  the  ears  are  not  used.  Some  good  plants 
are  lost  because  all  the  pollen  has  been  shed  and  has  lost  its 
viability  before  any  silks  appear.  This  tendency  to  protandry  is 
accentuated  in  some  inbred  lines.  Such  strains  could  be  main- 
tained by  sib-crossing  but  since  this  method  of  inbreeding  is  much 
less  effective  than  self-fertilization  in  bringing  about  homozygosity 
the  latter  system  has  been  rigidly  adhered  to.  In  this  way  sterility 
and  recessive  abnormalities  of  all  kinds  are  most  quickly  eliminated. 

SELECTION   OF   EARS   FOR   PLANTING. 

Each  hand-pollinated  plant  is  tagged  with  a  printed  form  upon 
which  notes  as  to  the  character  of  the  plants  in  the  field  and  the 
hand-pollinated  ears  when  mature  are  entered  as  follows: 

Pedigree  number  Color  and  markings  of  foliage 

Field  plot  number  Infection  on  plant 

Height  to  ear-bearing  node  Smut  on  ear 

Height  to  first  branch  on  tassel  Mold  on  ear 

Number  of  ears  containing  seed  Number  of    rows  of    grain  on  ear, 

Number  of  leaves  regularity  of  rows,  and  length  of 

Number  of  tillers  '  ear 

Posture,  whether  erect,  leaning,  Color  and  general  character  of  seeds 

bent,  broken  or  fallen  Color  and  shape  of  cob. 


*  A  method  of  pollinating  proposed  by  Jenkins  and  known  as  the 
"bottle  method"  was  also  tried.  Under  our  conditions  it  did  not  prove 
as  satisfactory  as  the  procedure  described  here. 


386 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


At  harvest  these  tags  are  transferred  to  the  hand-pollinated 
ears.  In  choosing  the  three  ears  for  planting  in  each  line,  from  the 
five  ears  pollinated,  the  characters  of  the  plants  in  the  field  as  well 
as  the  size  and  appearance  of  the  ears  are  taken  into  consideration, 
chief  attention  being  given  to  ability  to  stand  erect,  color  of  foliage, 
freedom  from  smut  and  other  infection  on  the  plant  and  ear  and 
absence  of  mold  on>the  ears. 

ELIMINATION    OF    SELF-FERTILIZED    LINES. 

In  all,  86  self-fertilized  lines  were    started,  distributed  among 


Figure  40.  Plant  bagged  for  hand  pollination.  Small  bags  can  be 
used  over  the  ear  shoot  and  the  tassel  bag  placed  on  the  ear  when  polli- 
nated.    Wire  clips  are  now  used  to  hold  the  bags  on  the  ear  and  tassel. 

the  four  varieties  as  follows:  From  Burwell's  Yellow  Flint 
number  40  there  were  18  ears  self -pollinated  in  1918,  ranked  and 
numbered  from  1  to  18  in  order  of  their  excellence  as  shown  in 
figure  35.  In  addition  to  these  there  were  14  ears  of  the  same 
variety  which  had  been  self -fertilized  in  1914  for  another  purpose 
and  not  used.  These  were  included  among  the  Bur^vell  strains 
with  the  variety  ntunber  30  to  distinguish  them  from  the  other 
strains  which  were  ranked  according  to  their  appearance.  The 
fact  that  these  ears  had  been  held  five  years  before  planting  has 
interest  in  connection  with  the  possible  elimination  of  abnormal- 


ELIMINATION    OF    SELF-FERTILIZED    LINES 


387 


ties  due  to  the  age  of  the  seed,  as  will  be  noted  later.  From  the 
Gold  Nugget  variety,  number  105,  twenty  lines  were  started 
(figure  36);  from  Century  Dent,  number  110,  eighteen  lines 
(figure  37),  and  from  Beardsley's  Leaming,  number  112,  sixteen 
lines  were  started  (figure  38). 

The  once  self-pollinated  ears  beginning  these  86  lines  were 
planted  in  1919  and  hand-pollinated  ears  were  obtained  from  all 
lines  except  one  in  Gold  Nugget  and  two  in  Century  Dent.  These 
failures  to  produce  seed  in  all  five  pollinations  in  each  line  may 
have  been  due  to  delayed  pollination  and  unfavorable  weather 
conditions.     But  since  good  ears  were  obtained  in  the  other  lines 


Figure  41.  Pollinating  corn.  Only  one  man  is  necessary'  for  this  opera- 
tion. Care  is  taken  not  to  touch  the  silks  or  the  inside  of  tassel  bag.  If 
the  silks  are  more  than  three  inches  long  they  are  cut  back  to  about  one 
inch  with  a  knife  sterilized  in  alcohol. 


it  is  fair  to  assume  that  these  lines  were  less  vigorous  or  for  some 
reason  were  not  as  able  to  reproduce  under  this  method  of  polhna- 
tion.  In  the  second  generation  two  more  lines  were  lost  because 
no  self -pollinated  seed  was  obtained.  In  the  third  generation 
four  lines  were  discontinued.  In  two  of  these  no  hand-pollinated 
ears  were  obtained,  and  the  other  two  were  so  badly  damaged  by 
■mold  that  they  were  discarded. 

In  the  fourth  generation  eleven  lines  were  eliminated.     Nine 
-were  discarded  because  they  were  so  \'ery  poor  and     unpromising 


388 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


that  it  was  thought  advisable  not  to  carry  them  further.  Some  of 
these  failed  to  produce  any  seed  on  any  plants.  All  of  the  hand- 
pollinated  ears  of  two  lines  proved  to  be  out-crossed,  due  possibly 
to  the  fact  that  the  bags  covering  the  ears  of  the  previous  genera- 
tion were  broken  and  allowed  foreign  pollen  to  enter.  By  the 
fifth  generation  practically  all  of  the  lines  had-  become  uniform 
and  stable.  All  that  had  survived  up  to  this  point  gave  promise 
of  being  able  to  continue  indefinitely  if  sufficient  effort  was  put 
forth  and  provided  the  season  was  not  too  unfavorable.  During 
the  course  of  the  five-year  selection  period  the  following  lines  were 
eliminated  for  various  reasons : 


Figure  42.     Self-fertilized  ears  showino;  defective  or  aborted  seeds. 


In  No.  30,  line  2  was  accidentally  lost. 
In  No.  40,  lines  2,  5,  11,  12,  17,  18  were  discarded. 
In  No.  105,  lines  1,  4,  5,  12,  19  were  lost  or  discarded. 
In  No.  110,  lines  8,  12,  13,  14  were  lost  or  discarded. 
In  No.  112,  lines  2,  5,  11,  13  were  discarded. 

In  all,  20  lines  were  not  continued  to  the  end  of  the  fifth  genera- 
tion. Three  of  these  were  accidentally  lost  thru  no  fault  of  their 
own.  The  others  were  too  poor  to  be  carried  along.  An  examina- 
tion of  the  original  ears  from  which  these  lines  came  (figures  35  to 
38)  shows  no  marked  relation  between  their  poor  behavior  and 
their  appearance  when  first  pollinated.  Dividing  each  lot  of  ears 
into  two  equal  groups  and  not  counting  the  three  lines  that  were 


ELIMINATION    OF    SELF-FERTILIZED    LINES 


389 


accidentally  lost,  we  find  that  seven  from  the  best  appearing  lines 
at  the  start  were  discarded  and  ten  from  the  poorest. 

The  original  plan  was  to  keep  all  lines  that  could  be  successfully 
propagated  even  though  they  became  extremely  poor.  It  was 
fully  appreciated  that  inbred  strains    may    themselves  be  very 


Figure  43.     Seedlings    lacking    chlorophyll     are     common     hereditary- 
variations  in  corn. 


undesirable  and  still  have  potentially  great  value  when  crossed 
with  other  strains.  For  this  reason  no  lines  were  discarded  unless 
the  amount  of  seed  produced  was  so  small  that  enough  plants  to 
permit  satisfactory  measurements  could  not  be  grown.  Many  lines 
were  continued  which  were  extremely  weak,  unproductive  and 
showed  markedly  undesirable  characters.     They  were  continued 


390 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN    266. 


to  compare  them  in  crossing  with  other  strains.  The  results  of 
these  comparisons  will  be  reported  in  a  later  pubhcation.  It  should 
be  emphasized  here  that  the  20  lines,  or  23  per  cent,  of  the  original 
number,  which  were  lost  or  discarded,  represent  for  the  most  part 
extremely  poor  and  undesirable  material  that  would  probably  be 
lost  in  any  selection  experiment.  By  growing  a  larger  number  of 
plants  in  order  to  give  a  greater  opportunity  for  selection  and  by 
hand-pollinating  a  larger  number  of  individuals  it  would  probably 
have  been  possible  to  continue  many  of  these  lines  and  some 
might  even  have  turned  out  to  be  good  strains  in  the  end.  Whether 
it  is  worth  while  to  work  more  intensively  with  a  few  lines  or 
expend  the  same  amount  of  time  on  a  larger  number  of  strains  less 
intensively  selected  is  one  of  the  most  important  problems  to  be 
considered. 


Figure.  44     Various  tj^pes  of  chlorophyll  deficiencies  found  in  inbred 
strains  of  corn. 


THE    PRODUCTION    OF   ABNORMALITIES 

An  examination  of  the  original  ears  after  the  first  self-fertilization 
(figures  35  to  38)  showed  eight  that  were  segregating  tor  small,  dull 
colored  seeds  that  were  clearly  abnormal.  These  recessive  seeds 
varied  on  different  ears  from  almost  entirely  empty  pericarps  to 
seeds  nearly  normal  in  size  but  shriveled  and  opaque  in  appear- 
ance, as  shown  in  figure  42.  These  aborted  seeds,  in  most  cases, 
failed  to  grow  and  those  which  did  germinate,  produced  abnormal 
seedlings  none  of  which  reached  maturity.  The  normal  seeds 
from  ears  showing  defectives  when  planted  produced  segregating 
ears  on  some  of  the  plants  in  the  following  generation.  In  addition, 
five  ears  which  were  not  clearly  segregating  in  the  first  generation 
produced  some  ears  with  abnormal  seeds  in  their  second  generation 
progenies.  It  has  since  been  found  that  this  defective  seed  condi- 
tion is  due  to  a  large  number  of  lethal  or  semi-lethal  factors  which 
are  hereditarily  distinct.  They  are  wideh'  distributed  in  all  kinds 
of  corn.     In  cross-pollinated  plants  only  a  few  of  these  abortive 


PRODUCTION    OF    ABNORMAfilTIES 


391 


Figure  45.     A  chlorophyll-deficient  dwarf  compared  to  a  normal 
plant  in  the  same  family. 


392 


CONNECTICU*r    EXPERIMENT    STATION 


BULLETIN    266. 


seeds  are  seen  on  any  ears  and  these  are  not  conspicuous.  It  is 
quite  possible  that  the  plants  carrying  these  factors  in  a  hetero- 
zygous condition  may  be  seriously  weakened  by  them  and  for 
that  reason  the  elimination  of  these  lethal  endosperm  factors  is 
probably  important. 

When  the  first  generation  self -fertilized  ears  were  grown,  chloro- 
phyll-deficient seedlings  appeared  as  Mendelian  recessives  in 
fifteen  lines.  Eight  of  these  segregated  for  white  seedlings,  one 
yellow,  three  yellowish  green  and  three  light  green.  These  abnor- 
mal seedlings  were  quite  distinct  and  most  of  them  died  as  soon 
as  the  food  stored  in  the  seeds  was  exhausted.  Several  distinct 
types  of  striped  and  variegated  plants  which  represent  various 
forms  of  chlorophyll  deficiency  were  observed  and  are  shown  in 
figure  44. 

Other  clear-cut  abnormalities  which  appeared  in  the  first  genera- 
tion as  recessive  segregates  were  golden  plants  in  four  lines, 
various  forms  of  dwarfs  in  three  lines,  sterile  tassels  which  pro- 


Figure  46.       Various  types  of  dwarfs  found  in  inbred  strains  of  corn. 

duced  no  pollen  in  five  lines.  Barren  plants  without  ears  and 
which  had  the  appearance  of  being  simple  Mendelian  recessives 
were  found  in  three  lines  but  the  inheritance  of  such  steiility 
factors  has  not  been  definitely  proven. 

In  addition  to  these  common  abnormalities  some  new  characters 
were  found  which  had  not  been  observed  in  other  material.  A  few 
plants  of  one  line  bore  ears  with  no  silks  and  such  plants  were 
therefore  entirely  sterile  in  the  pistillate  parts  as  shown  in  figure  47. 
Good  pollen  was  produced  and  when  crossed  on  to  normal  plants 
the  silkless  ears  reappeared  in  later  generations.  This  character 
was  not  found  until  the  second  generation.  It  may  have  occurred 
the  first  year  and  been  overlooked.  Another  strain  produced 
square  cobs  and  another  had  ears  with  many  silks  in  place  of  one 
for  each  seed.  This  latter  character  failed  to  reappear  in  later 
generations  and  apparently  was  not  inherited,  or  at  least  not  as  a 
simple  recessive.  IVIany  other  variations  from  normal  occurred. 
They  differed  in  degree  of  abnormality,  some  affecting  the  plants 
much  more  seriously  than  others. 


PRODUCTION    OF    ABNORMALITIES 


393 


In  twelve  lines  no  abnormalities  were  noted  in  the  first  two 
generations,  but  in  the  third  or  fourth  generation,  various  types 
appeared,  in  the  form  of  chlorophyll-deficient  seedlings,  striped 
and  variegated  plants,  dwarfs,  seedlings  with  tube  leaves 
instead  of  normally  flat,  and  plants  with  only  the  mid-ribs  in  place 
of  normal  leaves.  In  some  of  these  cases  recessive  segregates  may 
not  have  appeared  in  the  first  generations  on  account  of  elimination 
due  to  poor  germination  or  they  may  have  been  thinned  out  with 


Figure  47, 
family. 


Silkless  ears  compared  to  normal  specimens  from  the  same 


the  weaker  seedlings.  In  some  cases,  however,  there  seems  to  be 
no  question  that  they  are  due  either  to  original  mutations  or  to 
delayed  segregation  resulting  from  some  complicated  mode  of 
inheritance.  A  good  illustration  can  be  given  in  the  production 
of  the  narrow-leaved  plants  shown  in  figure  48.  Such  a  striking 
variation  as  this  could  not  be  easily  overlooked.  All  the  selected 
lines  were  carefully  examined  for  abnormalities  throughout  the 
season,  beginning  with  the  early  seedling  stage.  Narrow-leafed 
plants  were  first  observed  in  the  third  generation  in  lines  112-13 


394  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 


Figure  48.      Plants  with  narrow  leaves  occurred  in  two  inbred  lines. 


PRODUCTION    OF    ABNORMALITIES  395 

and  112-14.  All  three  progenies  of  112-13  produced  some  abnormal 
plants;  two,  nine  and  eleven  narrow-leafed  individuals  appearing 
in  the  different  progenies  in  a  total  of  about  25  plants  in  each. 
This  line  had  been  segregating  previously  for  dwarfs,  golden  plants, 
yellowish  seedlings  and  striped  dwarfs.  Line  112-14  produced  one 
narrow-leafed  plant  in  the  third  generation.  Though  only  normal 
plants  were  self -pollinated  in  the  third  generation,  all  of  the  fourth 
generation  plants  in  line  112-13  were  abnormal,  being  short  and 
with  streaked  and  wrinkled  leaves  varying  in  width  from  a  mere 
mid-rib  to  nearly  full  width.  The  plants  were  so  poor  that  no 
self -pollinated  ears  were  obtained  and  the  line  was  lost.  Line 
112-14  produced  no  narrow  leaves  in  the  fourth  generation.  All 
the  plants  were  described  as  uniform,  leafy  but  short  in  stature. 
In  the  fifth  generation  three  progenies,  all  from  ears  borne  on 
normal  plants  in  the  fourth  generation,  were  grown.  No  plants 
were  obtained  from  one  and  only  a  few  in  the  other  two.  All 
of  these  had  typical  narrow  leaves  and  were  badly  stunted.  They 
made  a  feeble  growth  and  produced  no  ears. 

Pollen  from  typical  narrow-leafed  plants  of  the  third  generation 
out-crossed  on  to  normal  plants  failed  to  show  any  abnormal 
plants  in  either  the  first  or  the  second  generation.  Five  self- 
fertilized  progenies  of  the  third  generation  were  grown  and  in 
about  30  plants  one  narrow-leafed  plant  was  found.  The  inheri- 
tance of  this  abnormality  is  not  understood. 

In  the  fourteen  lines  of  Burwell's  Flint  which  came  from  ears 
self -pollinated  in  1914  and  not  planted  until  1919  no  abnormalities 
of  any  kind  were  noted  in  the  first  two  generations.  In  the  third 
and  fourth  a  few  chlorophyll-deficient  seedlings,  striped  plants  and 
tube  leaves  appeared.  In  contrast  to  this  are  the  18  lines  of  the 
same  variety  self-pollinated  in  1918  and  planted  the  following 
year  which  segregated  the  first  generation  for  defective  seeds, 
dwarf  plants  and  chlorophyll-deficient  seedlings  in  five  lines.  Five 
other  lines  of  this  lot  were  so  poor  they  were  discarded,  while  none 
of  the  1914  lot  were  eliminated.  Though  the  number  of  lines  is 
too  few  to  be  conclusive  it  seems  that  the  delay  of  five  years  in 
planting  may  have  eliminated  many  abnormalities  by  the  death  of 
the  seeds  carrying  them.  A  germination  test  of  these  ears,  made 
in  1919,  showed  a  viability  ranging  from  10  to  100  per  cent.  Eight 
of  the  14  ears  germinated  90  per  cent,  or  less.  None  of  the  one 
year  old  self -pollinated  ears  of  the  same  variety  germinated  less 
than  85  per  cent,  and  only  two  were  less  than  95  per  cent.  There 
was  clearly  an  elimination  of-  seeds  in  the  five-year  resting  period 
and  this  could  easily  have  been  selective,  the  seeds  carrying  the 
recessive  abnormalities  being  less  viable.  If  this  is  proven  to  be 
the  case,  some  method  of  destroying  the  less  viable  seeds  such  as 
exposure  to  high  temperature,  alternate  germinating  and  dr}'-ing 
or  similar  harsh  treatment  may  be  an  effective  means  of  weeding 
out  defective  germplasm. 

Many  of  these  recessive  abnormalities  after  they  once  appeared. 


396  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 


^ 


^  J- 


J      .-"if 


Figure  49.      Representative   plants  of   three  flint  lines;  from 
top  to  bottom  they  are  40-4,  105-10,  and  105-20. 


PRODUCTION    OF    ABNORMALITIES  397 

kept  reappearing  in  the  following  generations,  but  were  finally 
eliminated,  in  every  case  except  one,  by  the  fifth  generation.  One 
line  which  was  vigorous  and  productive  and  quite  uniform  in  the 
fifth  generation  has  segregated  for  white  seedlings  in  CA^ery  genera- 
tion. Selection  of  progenies  has  usually  been  based  upon  produc- 
tiveness and  general  appearances  of  the  plants  without  regard  to 
whether  they  were  segregating  for  abnormalities  or  not. 

Out  of  the  original  86  lines  only  32  lines  or  37  per  cent,  showed 
no  clear-cut  recessive  abnormalities  during  the  five  generations 
they  were  self -fertilized.  As  stated  before,  13  lines  or  15  pei  cent, 
segregated  for  defective  seeds,  and  15  lines  or  17  per  cent,  for 
chlorophyll-deficient  seedlings.  Many  of  the  lines  had  several 
types  of  abnormality.  In  a  lot  of  575  self-fertilized  ears  from  six 
varieties  of  white  fiint  com  in  another  selection  experiment  there 
were  found  19  ears  or  a  little  more  than  3  per  cent,  segregating 
for  defective  seeds.  Of  these,  441  were  grown  and  40  lines  or  9 
per  cent,  were  found  to  be  segregating  for  chlorophyll-deficient 
seedlings.  Hutchison  self -fertilized  2,110  ears  from  a  large  number 
of  different  varieties  of  corn  common!}^  grown  in  various  parts  of 
the  country  and  found  3  per  cent,  segregating  for  defective  seeds 
and  36  per  cent,  for  various  seedling  characters,  of  which  the  greater 
nxrmber  were  chlorophyll  deficiencies. 

The  widespread  occurrence  of  these  recessive  abnormalities  is 
fully  established.  In  normally  cross-pollinated  plants  they  are 
comparatively  rare  in  appearance  since  they  are  present  as  reces- 
sives  in  the  heterozygous  condition.  To  what  extent,  if  any,  they 
reduce  growth  in  the  heterozygous  condition  has  not  been  estab- 
lished. Lindstrom  (1920)  suggests  that  in  eliminating  these 
recessive  abnormalities  many  desirable  factors  with  which  they 
are  linked  may  also  be  taken  out.  Since  these  recessives  are 
presumably  scattered  throughout  the  chromosomes  many  other 
factors  both  good  and  poor  will  be  taken  out  A^ath  them. 

It  has  been  argued  that  the  recessive  abnormalities  tend  to  be 
eliminated  by  natural  selection  except  in  those  cases  where  they 
happen  to  be  closely  linked  with  exceptionally  favorable  growth 
factors,  in  which  case  they  would  be  preserv^ed,  and  in  weeding 
them  out  the  factors  which  promote  growth  woiild  be  lost  with 
them.  The  only  answer  to  such  an  argument  is  to  see  what  the 
facts  are.  Twenty-five  lines  segregating  for  clear-cut  abnormalities 
gave  progenies  in  the  following  generation,  some  with  and  some 
without  the  recessives.  The  25  progenies  which  still  carried  the 
recessives  averaged  50.8  bushels  per  acre  yield  in  comparison  with 
50.4  bushels  for  the  25  progenies  grown  in  the  adjoining  rows,  and 
from  which  the  abnormalities  had  been  eliminated.  An  equally 
good  stand  was  obtained  in  each  case,  as  an  excess  of  seed  was 
planted  and  the  recessive  abnormalities  thinned  out.  The  differ- 
ence in  yield  in  the  two  lots  is  not  significant.  If  there  are  favorable 
gro\\^h  factors  in  the  segregating  progenies  which  are  not  present 


398  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266 


S^nipsisppp  ' 


% 


/V^t; 


/    X  „   '   "!/' 


Figure  50.     Representative  plants  of  three  earl}^  dent  lines;  from 
top  to  bottom  they  are  110-4,  6,  10. 


UNIFORMITY    AND    CONSTANCY  399 

in  the  non-segregating  progenies  from  the  same  grand-parental 
plant  they  have  no  more  effect  than  to  counterbalance  an}^  weaken- 
ing influence  that  the  recessive  abnormalities  may  have  in  the 
heterozygous  condition. 

Another  comparison  is  made  by  finding  the  average  per  cent, 
reduction  in  yield  of  all  segregating  lines  from  the  first  generation 
to  the  fifth  generation,  by  which  time  the  abnormalities  were 
eliminated.  This  reduction  was  found  to  be  57.1  per  cent,  com- 
pared to  the  reduction  of  58.1  for  all  lines  which  were  free  from 
abnormalities  at  the  start.  If  any  favorable  groA^ith  factors  were 
lost  when  the  recessive  characters  were  weeded  out,  their  departure 
caused  no  greater  reduction  in  yield  than  took  place  in  the  other 
material  from  which  no  abnormalities  were  removed. 

From  this  it  seems  evident  that  the  chances  are  no  greater  for 
good  factors  to  be  eliminated  than  poor  ones  and  with  other  things 
being  equal  it  seems  highly  desirable  to  take  out  these  clear-cut 
recessive  abnormalities.  In  fact  it  is  necessary,  in  most  cases,  to 
eliminate  all  lethal  and  semi-lethal  factors,  in  order  to  bring  the 
strains  to  uniformity. 

THE    APPROACH   TO    UNIFORMITY    AND    CONSTANCY. 

As  expected,  the  first  and  second  generations  were  quite 
variable  but  in  the  third  generation,  after  three  successive  self- 
fertilizations,  a  number  of  lines  became  fairly  uniform  in  height 
of  plant,  color  of  foliage  and  in  general  characteristics.  In  the 
fourth  generation  the  majority  of  the  lines  had  become  well  fixed 
in  their  type,  and  after  five  generations  all  of  the  selected  lines, 
with  a  few  exceptions,  were  alike  within  themselves.  This 
uniformity  was  apparent  in  the  plants  of  each  progeny  and  in  the 
similarity  among  the  several  progenies  of  the  same  line.  A  few 
lines  remained  variable  throughout  the  five  generations.  As  a 
rule  the  lines  that  showed  uniformity  in  the  third  generation  de- 
clined somewhat  in  size  and  yield  in  the  two  subsequent  generations. 
Practically  all  of  the  best  strains  can  be  picked  in  the  fifth  genera- 
tion. Many  of  them  can  be  recognized  in  the  fourth  and  a  few 
in  the  third.  However,  it  is  necessary  to  have  a  record  of  their 
performance  during  two  and  preferably  three  seasons  after  uni- 
formity is  reached  in  order  to  be  sure  that  they  are  fixed  in  their 
type.  Several  strains  that  were  considered  to  be  ^^ery  promising 
in  the  third  generation  declined  so  in  vigor  and  productiveness 
in  the  two  following  generations  that  they  were  much  inferior  to 
strains  that  had,  earlier,  been  far  less  promising.  On  the  other 
hand  a  few  of  the  most  vigorous  and  productive  lines  in  the  fourth 
and  fifth  generations  were  not  noted  as  being  promising  in  the 
third.  While  it  cannot  be  asserted  positively  that  strains  which 
are  uniform  and  good  in  appearance  during  the  fourth  and  fifth 
generations  will  maintain  themselves  without  further  reduction 
the  evidence  from  the  older  inbreeding  experiments  indicates  that 


400  CONNECTICUT    EXPERIMENT    STATION  BULLETIN   266. 


^-^l^     ,^      .^      '/%    \ 


7.,1  \-%h 


Figure   51.     Representative   plants   of   three    late 
dent  lines;  from  top  to  bottom  they  are:  112-1,  4,  9. 


DIFFERENCES    IN    SELECTED    LINES  401 

they  can  be  expected  to  maintain  their  level  of  vigor  without  much 
loss.  Therefore  in  carr^dng  out  a  selection  process  of  this  kind 
the  fourth  and  fifth  generations  are  the  most  important  in 
affording  an  opportunity  to  pick  the  best-appearing  self -fertilized 
strains. 

The  selection  process  was  carried  out  with  the  aim  of  securing 
the  most  vigorous  and  productive  inbred  strains,  uniform  and 
fixed  in  their  t3^pe  so  that  their  good  qualities  could  be  maintained 
indefinitely.  For  this  purpose  five  generations  of  self-fertilization 
are  necessary  in  most  cases. 

Differences  in  the  Selected  Lines. 

In  the  fourth  generation  all  of  the  selected  lines  had  become 
strikingly  differentiated.  Differences  in  height,  color  of  foliage, 
size  and  shape  of  ears  made  each  line  distinct  from  every  other  line. 
In  the  Burwell  Flint  lines  differences  in  average  height  ranged  from 
51  to  98  inches,  in  the  Gold  Nugget  lines  from  44  to  84,  in  the 
Century  Dent  from  44  to  76  and  in  the  Beardsley's  Learning  lines 
from  54  to  100.  Color  of  foliage  varied  from  ver\^  dark  bluish 
green,  through  all  gradations  in  shade  to  light  green  and  yellowish 
green.  In  some  lines  the  leaves  were  streaked  with  alternate 
rows  of  light  and  dark  tissue.  Various  forms  of  fine  and  coarse 
flecking  and  mottHng  of  the  leaves  were  a  regular  feature  of  some 
strains  while  others  were  entirely  free  from  this  ph}'siological  irreg- 
ularity of  the  chlorophyll. 

The  flint  strains  were  most  noticeably  different  in  number  of 
tillers.  A  number  produced  no  large  tillers  and  some  had  only  a 
very  few  inconspicuous  shoots  from  the  base  of  the  plants.  Others 
branched  very  freely,  producing  many  large  branches  on  every 
plant.  Alany  of  these  were  as  large  as  the  main  stalk  and  bore  ears. 
Some  strains  regularly  produced  seeds  in  the  tassels  on  nearly  all 
plants  while  others  never  did  this. 

The  ability  to  stand  erect  throughout  the  season  is  one  feature 
that  has  been  carefully  selected  for  in  all  lines.  Marked  differ- 
ences in  this  respect  were  sho'WTi,  being  greater  in  some  seasons 
than  in  others.  Certain  lines  regularly  went  down  sometime 
during  the  latter  part  of  the  season  while  others  stood  stiffly  erect 
up  to  maturity.  Equally  pronounced  differences  in  time  of 
flowering  are  sho^^^l  by  the  lines  derived  from  the  same  variety. 
Most  of  the  lines  matured  satisfactorily  every  season  while  others 
were  so  late  as  to  be  barely  able  to  ripen  seed.  The  weakening 
effect  of  inbreeding  delays  maturity  in  all  lines  but  in  spite  of  this 
some  were  earlier  in  ripening  than  the  variety  from  which  they 
were  derived.  Along  with  these  diff'erences  in  maturity  were 
great  dissimilarities  in  character  of  the  grain.  The  seeds  of  some 
were  hard,  translucent  and  bright  colored;  others  were  soft,  dull 
colored  and  in  some  lines  regularly  moldy. 


402  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 


Figure  52.  Representative  ears  of  four 
productive  Burwell  Flint  lines;  from  top  to 
bottom  they  are:  30-19,  40-1,  7,  S. 


DIFFERENCES    IN    SELECTED    LINES 


403 


Figure  53.  Representative  ears  of  four  unpro- 
ductive Burwell  Flint  lines;  from  top  to  bottom 
they  are:  30-5,  6,  40-15,  16. 


404  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 

The  features  named  are  the  more  striking  ones.  Differences  in 
structural  details  are  brought  out  in  the  accompanying  illustra- 
tions showing  the  plants  and  ears  of  some  of  the  selected  lines  in  the 
fourth  generation  (figures  49  to  57).  In  details  of  structure  and 
arrangement  of  parts  the  lines  are  so  distinct  that  they  can  usually 
be  easily  recognized  in  the  field  and  after  harvest.  In  a  few 
features  certain  strains  may  be  alike.  Some  strains  have  similar 
plants  but  differ  decidedly  in  ear  structure.  In  others  the  ears 
are  somewhat  similar  but  are  borne  on  markedly  different  plants. 
For  the  most  part  the  differences  are  far  more  obvious  than  the 
similarities. 

Susceptibility  to  Disease. 

The  most  common  diseases  with  which  com  has  to  contend  in 
Connecticut  are  smut  (Ustilago) ,  leaf  blight  (Helminthosporium) , 
and  various  -root,  stalk  and  ear  roots  (Diplodea,  Gibberella  and 
other  forms  of  Fusarium).  Marked  differences  in  smut  infection 
were  shown.  Two  lines  105-14  and  110-17  showed  no  smut 
infection  on  any  plant  in  any  progeny  during  the  five  generations 
they  were  grown.  Eleven  strains  had  no  more  than  one  plant 
affected  in  any  one  year  throughout  the  same  period.  The  place 
on  the  plant  where  the  smut  balls  appeared  was  usually  quite 
characteristic,  some  strains  having  them  on  the  basal  nodes,  others 
at  the  ear  node,  still  others  on  the  leaves  or  tassels.  In  some  lines 
numerous  light  infections  on  the  plant  or  ears  were  shown  which 
apparent^  did  not  do  any  serious  damage.  Other  strains  had 
many  plants  badly  injured  and  sometimes  killed  outright  during 
mid-season.  The  most  striking  case  of  segregation  of  suscepti- 
bility to  parasitism  by  the  smut  fungus  occured  in  line  110-3.  In 
the  first  generation  four  per  cent,  of  the  plants  were  smutted.  In 
the  second  three  progenies  were  grown  having  twelve  plants  in 
each.  In  one  progeny  none  of  the  plants  had  any  indication  of 
smut  infection.  In  another  all  of  the  plants  were  smutted  and 
most  of  them  were  killed  during  the  middle  of  the  summer.  In 
the  third  progeny  27  per  cent,  of  the  plants  were  attacked.  The 
original  seed  of  the  two  strikingly  different  progenies  was  planted 
again  the  following  year  with  the  result  that  out  of  57  plants  of  the 
resistant  progeny,  only  one  plant  o^  1.7  per  cent,  was  infected. 
The  smutted  lot  had  14  plants  infected  out  of  31  grown,  or  45.2 
per  cent.  In  the  next  generation  no  smutted  plants  were  seen  in 
the  one  line  and  65.6  per  cent,  in  the  other.  Marked  differences 
were  shown  in  the  seeds  of  the  two  lines.  Plants  of  the  susceptible 
line  were  extremely  weak  but  the  seeds  were  normal  in  appearance. 
However  the  germination  of  these  seeds  was  poor  and  in  the  fifth 
generation  no  plants  were  obtained.  The  resistant  line  produced 
more  vigorous  plants  having  a  noticeably  darker  green  color. 
All  of  the  seeds  produced  on  these  plants  were  distinctly  abnormal . 
When  dry  they  were  shriveled  and  discolored  although  not  showing 


SUSCEPTIBILITY    TO    DISEASE 


405 


Figure  54.  Representative  ears  of  four  Century 
Dent  lines:  from  top  to  bottom  they  are:  110-3, 
4,  5,  10. 


406  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 

any  of  the  usual  molds.  Ears  of  this  line  are  shown  in  figure  54. 
In  spite  of  their  unfavorable  appearance  some  of  the  seeds  germ- 
inate and  the  plants  produced  are  about  as  good  as  the  average 
inbred  strain  of  the  same  variety. 

None  of  the  smut-free  lines  were  outstandingly  good  in  other 
respects  and  some  of  the  most  vigorous  and  productive  strains  now 
regularly  show  a  high  percentage  of  smut  infection.  The  smut -free 
or  low-smut  strains  may  have  value  in  crossing  with  other  strains 
which  have  good  qualities  but  are  lacking  in  smut  resistance. 

The  growing  season  of  1922  was  unusually  wet  and  the  selected 
lines  then  in  the  fourth  generation  showed  very  pronounced 
differences  in  the  amount  and  severity  of  infection  of  Helmintho- 
sporitun.  This  organism,  which  is  seldom  injurious  to  ordinary 
cross-pollinated  corn,  readily  attacks  many  inbred  plants  and  on 
some  completely  kills  the  leaves  after  seed  formation  begins.  Leaf 
blighting  due  to  this  organism  had  been  noted  each  year  in  some 
lines  but  in  the  wet  season  of  1922  it  was  particularly  injurious. 
Seventeen  of  the  eighty-six  lines  showed  heavy  infection.  Some 
of  them  lost  all  their  foliage  prematurely  and  the  ears  were  badly 
stunted,  the  grains  being  small  and  poorly  developed.  Some  of 
the  most  vigorous  and  productive  strains  in  former  years  were  so 
injured  in  this  way  as  to  give  them  a  very  low  rating.  The  follow- 
ing year  was  unusually  dry.  Very  little  damage  from  this  cause 
was  seen,  but  the  effect  of  the  drought  on  different  strains  was  very 
striking.  Some  strains  which  had  always  before  produced  green 
luxirriant  foliage  had  their  leaves  killed  at  the  sides  and  tips  by  the 
dry  heat  and  were  unproductive  for  that  reason.  Most  of  the 
strains  which  had  been  badly  injured  by  leaf  infection  in  the  wet 
season  were  beautifully  green  throughout  the  dry  period  of  1923 
and  were  among  the  best  appearing  and  most  promising  of  all  the 
selected  lines.  These  marked  differences  in  different  seasons  makes 
it  extremely  difficult  to  judge  the  value  of  inbred  strains  and  makes 
it  necessary  to  test  them  during  several  years  after  they  have 
become  uniform  and  fixed  in  type. 

The  investigations  of  Hoffer,  Holbert  and  others  have  empha- 
sized the  importance  of  the  root,  stalk  and  ear  rot  organisms 
attacking  corn.  The  results  of  the  earlier  inbreeding  experiments 
indicated  that  marked  differences  would  be  found  among  inbred 
plants  to  resist  infection.  Throughout  the  selection  experiment 
great  importance  was  placed  on  the  ability  of  the  plants  to  stand 
erect  throughout  the  season  and  have  the  ears  free  from  any  in- 
dication of  mold.  Fallen  plants  or  moldy  ears  were  avoided  when- 
ever possible.  The  most  outstanding  differences  in  ability  to 
stand  erect  and  in  freedom  from  mold  on  the  ears,  were  seen  in  the 
third  and  later  generations.  In  1922,  a  wet  season,  four  lines 
(30-6,  105-20,  110-2,  110-15)  had  all  the  plants  of  all  three  progenies 
erect  throughout  the  season.  This  same  vear  twelve  lines  (30-8, 
30-9,    105-3,    105-18,    110-1,    110-2,    110-6^   110-7,    110-18,    112-6, 


SUSCEPTIBILITY    TO    DISEASE 


407 


Figure  55.  Representative  ears  of  four  Gold 
Nugget  flint  lines;  from  top  to  bottom  they  are: 
105-3,  10,-17,  20. 


408  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 


Figure  56.  Representative  ears  of  four  produc- 
tive Beardsley's  Learning  lines;  from  top  to  bottom 
they  are:  112-1,4,  6,  9. 


SUSCEPTIBILITY    TO    DISEASE 


409 


1  = 

w 

/ 

^C!S< 

<  • 

I*-  , 

1? 


Figure  57.  Representative  ears  of  four  unpro- 
ductive Beardslev's  Learning  lines;  from  top  to 
bottom:  112-3,  10,  14,  15. 


410  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 

112-7,  112-8)  produced  no  moldy  ears.  Only  one  line  110-2  had  all 
plants  erect  with  ears  free  from  mold.  In  the  first  generation  this 
line  had  four  per  cent,  of  moldy  ears  and  ten  per  cent,  of  fallen 
plants  but  no  smut.  In  the  second  generation  there  were  ten  per 
cent,  moldy  ears,  ten  per  cent,  fallen  plants  and  no  plants  showing 
smut  infection.  In  the  third,  fourth  and  fifth  generations  there 
was  no  mold,  smut  or  fallen  plants  on  the  three  progenies  grown 
each  year.  This  strain  is  also  productive  for  the  variety,  although 
surpassed  in  this  respect  by  several  other  strains.  The  seeds  are 
hard  and  bright  but  very  pale  yellow  in  color  and  almost  white  on 
top. 

In  contrast  to  this  is  line  105-20  with  all  the  plants  erect  in  the 
second,  third  and  foirrth  generations  but  with  29,  17  and  44  per 
cent,  of  moldy  ears  in  the  same  years.  On  the  other  hand,  40-8 
had  all  of  the  plants  fallen  in  two  progenies  of  the  fourth  generation 
and  no  moldy  ears.  To  complete  the  combinations  112-11  had  87 
per  cent,  of  the  plants  on  the  ground  in  1922  and  67  per  cent,  of 
ears  moldy. 

Criterions  of  Selection. 

At  the  beginning  of  the  selection  experiment  the  plan  as  previous- 
ly stated  was  to  self -pollinate  five  plants  in  each  line  and  to  select 
three  of  the  best  self -fertilized  ears  for  planting  the  following  year. 
Even  when  these  ears  differed  greatly  in  appearance  no  consistent 
differences  were  noted  in  the  progenies  grown  from  them.  The 
coefficient  of  association  between  the  appearance  of  the  ear  and 
yield  of  the  different  progenies  within  several  lines  is- — .18.  This 
indicates  that  self -pollinating  a  large  number  of  ears  in  order  to 
make  more  extensive  selection  of  desirable  looking  ears  is  of  doubt- 
ful value.  Of  the  three  progenies  grown  only  one  was  to  be  chosen 
to  continue  the  line,  the  other  two  not  being  pollinated.  It  was 
soon  noted,  however,  that  there  was  very  little  relation  between  the 
appearance  of  the  progenies  at  the  time  of  bagging  and  their  pro- 
duction of  grain  and  the  general  appearance  of  their  plants  at 
harvest.  The  coefficient  of  association  between  the  appearance 
of  the  plants  at  pollinating  time  and  the  yield  of  the  different 
progenies  within  the  several  lines  is  — -.28.  Seedlings  were  groum 
in  the  greenhouse  and  their  weight  and  height  after  thirty  days  of 
growth  were  compared  with  the  yield  of  the  same  progenies  in  the 
field.  The  third  and  fourth  generations  showed  that  those  prog- 
enies that  had  the  tallest  seedlings  yielded  1.6  bushels  per  acre 
more  than  the  other  progenies  in  the  same  lines.  This  difference  is 
hardly  enough  to  make  a  selection  of  the  progenies  on  this  basis 
worth  while. 

Since  there  is  no  appreciable  correlation  between  the  characters 
of  the  seed-  ear,  weight  of  seed,  size  of  the  seedling,  or  the  appear- 
ance of  the  plants  at  pollinating  time  and  production  of  grain  the 
only  selection  of  progenies  that  can  be  made  with  any  degree  of 


CLASSIFICATION    OF    SELECTED    LINES 


411 


effectiveness  is  at  maturity.  Here  also  yield  is  highly  influenced 
by  the  amount  of  heterozygosity  remaining.  In  some  lines  there 
are  more  homozygous  combinations  than  in  others  and  they  are 
correspondingly  less  vigorous  and  productive  although  they  may  be 
potentially  more  desirable.  For  this  reason  final  judgment  must 
be  left  until  the  plants  are  reduced  to  uniformity  and  constancy. 
Hence  it  is  interesting  to  note  what  resemblance  the  resulting 
inbred  strains,  when  finally  reduced  to  imiformity  and  fixity  of 
type,  have  to  the  same  strains  in  the  first  generations  of  inbreeding. 

Classification  of  Selected  Lines. 

Taking  into  consideration  all  features  of  these  selected  lines  as 
they  grow  in  the  field  and  after  han^est  in  the  fourth  and  fifth 
generations  and  giving  most  importance  to  the  production  of 
bright  sound  grain,  the  four  outstanding  good  and  poor  strains  in 
each  variety  are  listed  as  follows,  with  their  yields  in  bushels  per 
acre  in  the  fifth  generation  compared  with  that  of  the  original 
variety  grown  the  same  ^^ear: 


Bur-well's  Flint  51.2 


Good  Lines 

Poor 

Lines 

Number 

Yield 

Number 

Yield 

30-5 

12.2 

30-10 

44.2 

30-19 

15.3 

30-18 

18.3 

40-4 

33.6 

40-3 

35.1 

40-8 

25.9 

40-16 

24.4 

Gold 

Nugget  54.0 

Good  Lines 

Poor 

Lines 

Number 

Yield 

Number 

Yield 

105-11 

29.0 

105-3 

9.2 

105-15 

33.6 

105-8 

13.7 

105-17 

22.9 

105-13 

7.6 

105-20 

10.7 

105-16 

29.0 

Century  Dent 

48.3 

Good  Lines 

Poor 

Lines 

Number 

Yield 

Number 

Yield 

110-2 

15.3 

110-1 

28.9 

110-4 

16.8 

110-9 

4.6 

110-5 

10.7 

110-15 

1.5 

110-10 

19.8 

110-17 

12.2 

Beardsley' 

s  Leaming  49.5 

Good  Lines 

Poor 

Lines 

Number 

Yield 

Number 

Yield 

112-1 

42.7 

112-3 

10.7 

112-6 

27.5 

112-7 

21.4 

112-9 

33.6 

112-14 

.0 

112-12 

12  2 

. 112-16 

9.2 

412 


CONNECTICUT    EXPERIMENT    STATION 


BULLETIN   266. 


This  is  purely  an  arbitrary  classification  based  upon  the  general 
appearance  of  the  plants  and  ears.  Some  of  the  poor  lines  yielded 
more  than  the  good  lines  but  produced  a  very  poor  quality  of  grain. 
The  original  ears  from  which  these  lines  descended  (figures  35  to  38) 
show  that  there  is  no  relation  between  the  good  and  poor  strains 
after  uniformity  was  attained  and  the  appearance  of  the  seed  ears 
from  which  they  came.  Low  and  high  numbers  are  represented 
about  equally  in  the  good  and  poor  strains. 

Correlation  Betwee^t  the  First  and  Last  Generations. 

In  order  to  find  out  whether  the  elimination  of  the  poor  lines  at 
the  beginning  of  the  inbreeding  period  is  advisable,  the  correlation 

Table  VIII, 

CoeflEicients  of  association  between  early  and  later  generations  of  self- 
fertilized  corn. 

Generations  Compared  1-4  1-4  1-5  2-5  1-4 

Variety  Height  Mold  Tillers  Smut  Yield 

Burwell's  Flint 60  .89  .64  —.08  0 

Gold  Nugget 35  0  .38  .38  .14 

Century  Dent 80  .80  —.72  .72  .28 

Beardsley's  Learning 95  .38  .50  .20  .50 

Ave.  Flints 50  .65  .55  .10  .05 

Ave.  Dents 89  .63  .17  .52  .38 

Average 71  .64  .27  .27  .19 

between  the  behavior  of  the  plants  in  the  first  generation  and  the 
last  generation  has  been  worked  out  for  the  most  important 
characters.     In  Table  VIII  are  shown  the  coefficients  of  association 


^ 

1 

— 

I  ■ 


I 


I 


i 


r 


FIRST        FOURTH 

HEIGHT 


FIRST  FOURTH 

MOLD 


FIRST         FIFTH 

TILLERS 


SFCCND       FIFTH 
SMUT 


FIRST  FOURTH 

YIETLD 


Figure  58.  Diagram  representing  the  average  of  the  upper  and  lower 
groups  in  the  first  generations  and  the  average  of  the  same  lines  in  the 
last  generations,  based  on  the  data  in  Table  IX. 


between  the  first  or  second  inbred  generation  and  the  fourth  or 
fifth  for  height  of  plant,  per  cent,  moldy  ears,  number  of  tillers, 
per  cent  smutted  plants,  and  yield  of  grain.  The  fifth  generation, 
grown  in  1923,  was  so  variable  on  account  of  the  extremely  dry 
season  afTecting  different  parts  of  the  field  unevenly  that  the 
coefficients  for  height  and  yield  are  based  on  the  first  and  fourth 
generations.     There  was  very  little  smut  infection  in  the  first 


CORRELATION  BETWEEN  GENERATIONS 


413 


generation  and  practically  no  mold  in  the  fifth  so  that  the  coefficient 
for  per  cent,  smut  is  based  upon  the  second  and  fifth  generations 
and  for  per  cent,  mold  upon  the  first  and  fourth. 

The  figures  show  a  fairly  high  association  for  height  of  plant  and 
moldy  ears.  This  means  that  by  selecting  the  highest  lines  in  the 
first  generation  the  resulting  inbred  strains  in  the  fourth  generation 
would  tend  to  be  taller  than  the  average.  Similarly,  by  selecting 
lines  at  the  start  that  were  free  from  mold,  inbred  strains  could 


GENERATIONS 

Figure  59.  Graph  showing  the  behavior  of  two  lines  with  respect  to 
height  during  four  generations  of  self-fertihzation,  selected  for  vigor  and 
productiveness  but  not  for  height.  From  one  to  three  progenies  are 
grown  in  each  generation. 


finally  be  attained  that  would  on  the  average  be  freer  from  moldy 
ears  than  other  strains  which  showed  more  mold  at  the  start. 
This  relation  does  not  hold  so  well  for  the  other  characters ;  number 
of  tillers  and  per  cent.  smut.  For  these  the  coefficients  are  low 
and  in  two  of  the  varieties  a  negative  correlation  is  shown.  This 
means  that  lines  without  tillers  and  showing  low  smut  infection 
may  be  obtained  from  plants  at  the  start  which  have  tillers  and  are 
susceptible  to  smut  infection. 


414  CONNECTICUT    EXPERIMENT    STATION  BULLETIN   266. 

Another  method  of  bringing  out  the  relation  between  the  several 
lines  at  the  start  and  at  the  end  of  the  selection  period  is  to  separate 
all  the  lines  of  each  variety  into  the  upper  and  lower  halves,  with 
respect  to  the  characters  studied,  in  the  first  generation  and  then 
compare  the  average  of  these  two  groups  with  the  averages  of  the 
same  lines  after  being  inbred  for  four  or  five  generations.  This  has 
been  done  in  Table  IX,  making  the  separation  within  each  variety 
into  equal  sized  groups  in  the  first  generation.  Thus  the  basis  for 
separating  the  groups  is  the  median  instead  of  the  mean.  The 
results  are  stmimed  up  graphically  in  Figure  58.  It  will  be  seen 
that  the  relative  position  of  the  upper  and  lower  halves  remains 
nearly  the  same  at  the  end  of  the  period  of  selection  as  at  the 


Table  IX. 

The  relative  position  of  the  same  self-fertiUzed  Unes  at  the 
and  at  the  end  of  the  period  of  selection. 

;  beginning 

Characters  measured 

Groups 

No.  of  Strains 
First            Last 

Average 
First       Last 

Relative 
First       Last 

Height  of  plant  in  inches 
in  the  first  and  fourth 
generations 

High 
Low 

36 
36 

94 
96 

81 
70 

70 
61 

100 

87 

100 

87 

Per  cent,  of  moldy  ears 
in  the  first  and  fourth 
generations 

High 
Low 

36 
35 

88 
95 

IS 
5 

15 

8 

100 
29 

100 
57 

Number  of  tillers  per  plant 
in    the     first    and    fifth 
generations 

High 
Low 

32 
33 

91 

94 

.9 
.3 

1.2 

.8 

100 
36 

100 
67 

Per  cent,    of    plants    with 
smut  in  the  second  and 
fifth  generations 

High 
Low 

32 
33 

90 
93 

14 
1 

12 

7 

100 
9 

100 
59 

Yield  of  grain  in  bu.  per 
acre    in    the    first  and 
fourth  generations 

High 
Low 

31 
31 

84 
82 

81 
52 

44 
41 

100 
64 

100 
93 

beginning  for  such  characters  as  height  of  plant,  number  of  tillers, 
per  cent,  of  moldy  ears  and  smutted  plants  although  the  difi^erences 
are  generally  less  at  the  close  than  at  the  start.  This  tendency  to 
change  during  the  period  of  inbreeding  is  most  marked  for  3deld  of 
grain.  In  this  respect  the  high  and  low  groups  are  ver}^  nearly 
ahke  at  the  end  of  the  selection  period  in  spite  of  the  fact  that  all 
along  attention  has  been  given  to  productiveness.  These  results 
indicate  that  it  is  unwise  to  eliminate  the  unproductive  strains  in 
the  first  generations,  as  from  them  lines  may  be  obtained  that  are  as 
productive  as  those  from  high  yielders  at  the  start.  Other  char- 
acters can  apparently  be  somewhat  more  surely  selected  for  at  the 
beginning  of  the  inbreeding  period.  If  such  characters  as  freedom 
from  mold  and  smut  are  of  chief  importance  it  might  be  advisable 
to  eliminate  those  lines  which  show  much  mold  and  smut  in  the  first 
inbred  y:enerations. 


LIMITING   FACTORS  415 

The  general  tendency  for  some  of  the  lines  to  hold  the  same 
relative  position  throughout  the  process  of  selection  is  illustrated 
by  the  height  of  plant  of  two  lines  shown  graphically  in  figure  59. 
In  the  first  inbred  generation  the  two  lines  averaged  69  and  77 
inches  in  height.  In  the  second  generation  two  progenies  over- 
lapped but  from  then  on  they  were  clearly  distinct,  the  difference 
in  height  increasing  until  the  end  of  the  selection  period.  The 
same  result  is  shown  in  the  average  number  of  tillers  per  plant  of 
two  other  lines  as  brought  out  in  figure  60.  Differing  at  the  start 
the  two  lines  remained  distinctly  different  in  all  their  progenies 
throughout  the  period  of  inbreeding.  In  marked  contrast  to  this 
is  the  result  shown  graphically  in  figure  61.  Two  fines  differing 
noticeably  in  their  nimiber  of  tillers  changed  positions  so  that  in 


3 
GENERATIONS 

Figure  60.  Graph  showing  the  behavior  of  two  lines  with  respect  to 
tillering  during  five  generations  of  self-fertilization,  selected  for  vigor  and 
productiveness  but  not  for  the  number  of  tillers.  The  relative  position 
of  these  two  lines  remained  the  same. 

the  end  the  few  tiller  strain  at  the  start  averaged  more  tillers  on  all 
progenies  than  did  the  many  tiller  strain.  Similarly  two  strains 
which  were  alike  in  this  respect  at  the  start  became  extremely 
different  as  uniformity  and  constancy  was  reached,  as  shown  in 
figure  62. 

Limiting  Factors. 

In  planning  and  carrying  out  a  selection  program  the  best 
procedure  will  depend  upon  the  number  of  plants  which  can  be 
grown  and  the  number  of  hand  pollinations  which  can  be  made  in  a 
season.  Where  the  facilities  available  for  artificial  pollination  is 
the  limiting  factor,  and  this  is  usuall}'  the  case,  the  best  procedure 


416  CONNECTICUT    EXPERIMENT    STATION  BULLETIN    266. 

is  to  self-pollinate  just  enough  plants  to  continue  as  many  lines 
as  possible  until  a  reasonable  degree  of  homozygosity  is  reached. 
If  the  amount  of  land  available  to  grow  the  plants  is  the  limiting 
factor  it  would  be  better  to  pollinate  a  larger  number  of  plants 
within  each  line,  although  extensive  selection  within  a  progeny 
has  been  shown  to  have  little  value,  as  the  better  individuals  are 
almost  certain  to  be  more  heterozygous,  making  it  difhcult  to  arrive 
at  their  true  value.  More  attention  should  be  paid  to  increasing 
the  number  of  progenies  within  the  more  desirable  appearing  Hnes, 
basing  selection  on  their  behavior  throughout  the  season  and  their 
uniformity  and  productiveness  at  maturity. 

The  method  now  being  used  at  this  station  is  to  grow  three 
progenies  in  each  line  and  to  pollinate  two  plants  in  each  progeny. 
On  the  basis  of  the  general  appearance  of  the  plants  in  the  field  and 


CrNCRATIOINS 

Figure  61.  Graph  showing  two  lines  which  differed  in  the  amount  of 
tillering  and  which  changed  positions  during  the  five  generations  of  self- 
fertilization. 

their  productiveness  at  maturity  the  best  and  second  best  progenies 
are  noted  where  there  is  an  appreciable  difference.  Two  ears  from 
the  best  progeny  and  one  from  the  second  best  are  used  for  planting 
the  following  year.  If  no  differences  are  shown,  one  ear  from  each 
of  the  three  is  planted.  This  procedure  is  based  upon  the  results 
in  the  five-year  selection  experiment  described  above  in  which  no 
reliable  criterions  of  selection  were  found  which  could  be  used 
before  the  time  of  pollination.  It  is  still  provisional  and  will  be 
modified  as  future  experience  justifies.  It  is  possible  that  better 
results  can  be  obtained  by  paying  still  less  attention  to  selection 
during  the  reduction  period  than  the  method  outlined.  By  expend- 
ing the  same  amount  of  time  and  effort  on  more  lines,  growing  only 
one  progeny  in  each  generation  and  pollinating  only  enough  plants 
to  insure  the  perpetuation  of  the  strain  until  uniformity  and 
constancy  are  reached,  more  diverse  material  would  be  available 


CONCLUSION  417 

from  which  to  select  the  best  inbred  strains.  In  this  procedure 
there  would  be  the  possibility,  and  even  probability,  of  missing 
altogether  valuable  material  which  might  exist  in  some  lines. 
However,  since  it  has  been  shown  that  many  of  the  lines  change 
greatly  during  the  reduction  process,  selection  during  this  period 
will  always  be  somewhat  ineffective.  From  a  theoretical  stand- 
point the  best  method  is  the  one  which  will  produce  the  largest 
number  of  fixed  strains  from  which  to  choose  the  ones  best  suited 
to  the  purpose  for  which  they  are  to  be  used. 

In  this  connection  one  further  point  should  be  mentioned.  When- 
ever any  particularly  outstandingly  good  strain  has  been  obtained 
there  is  the  possibility  that  still  better  material  may  exist  in  that 
strain  in  the  earlier  generations.     This  would  indicate  that  it 


GENERATIONS 

Figure  62.  Graph  showing  two  lines  which  showed  the  same  amount 
of  tillering  at  the  start  but  differed  widely  at  the  end. 

might  be  well  worth  while  to  go  back  to  the  earlier  generations  and 
grow  as  much  of  this  material  as  possible  from  the  remaining  seed 
in  order  to  obtain  the  very  best  gemiplasm  available  in  this  strain. 
In  fact,  this  procedure  has  already  been  followed  with  several  of  the 
more  promising  lines  and  it  has  been  possible  to  isolate  new  strains 
which  are  distinctly  superior  in  some  respects  to  the  old  ones. 

Conclusion. 

The  one  fact  that  stands  out  from  the  results  secured  in  this 
selection  experiment  is  that  there  is  no  single  criterion  by  which 
high-yielding  strains  can  be  obtained.  During  the  process  of 
inbreeding,  with  the  resulting  segregation  and  recombination  and 
the  automatic  elimination  of  heterozygous  combinations  of  factors, 
selection  for  particular   characters   is   somewhat   effective.     By 


418  CONNECTICUT   EXPERIMENT    STATION  BULLETIN    266. 

choosing  tall  plants  as  progenitors  in  each  generation  tall  strains 
can  be  produced.  By  selecting  plants  free  from  tillers,  strains  with  , 
few  tillers  can  be  obtained.  Similarly,  freedom  from  disease  in- 
fection, as  far  as  resistance  is  inherited,  can  be  expected  by  selecting 
during  the  reduction  period  only  those  plants  which  show  no 
infection  in  fields  where  infection  is  present.  Even  with  these 
characters  the  association  is  far  from  complete.  But  productive- 
ness, yield  of  grain,  which  sums  up  the  plant's  entire  energies  shows 
no  such  simple  relation.  High  yielding  strains  may  come,  and 
have  come,  from  plants  which  are  poor  producers.  Promising 
strains  during  the  first  generations  may  be  very  unproductive  or 
undesirable  in  some  respect  when  finally  reduced  to  uniformity  and 
constancy.  This  emphasizes  the  fact  that  effective  selection  must 
be  based  upon  the  performance  of  the  plants  after  homozygosity 
is  attained. 

LITERATURE  CITED. 

Cummings,  M.  B.,  and  Stone,  W.  C,  1921.    Yield  and  quality  in  Hubbard 

squash.     Vermont  A.  E.  S.  Bull.  222. 
East,  E.  M.,  1908.     Inbreeding  in  corn.     Report  Conn.  A.  E.  S.  1907- 

1908. 

1909.      The  distinction  between  development  and  heredity 

in  inbreeding.     Amer.  Nat.  43:  173-181. 
East,  E.  M.,  and  Hayes,  H.  K.,  1912.     Heterozygosis  in  evolution  and  in 

plant  breeding.     U.  S.  Dept.  Agr.,  Bur.  P.  I.  Bull.  243. 
Eyster,  W.  H.  1924.      A  primitive  sporophyte  in  maize.      Amer.  Jour.  Bot. 

11:7-14. 
Hayes,  H.  K.,and  Brewbaker,  H.  E.  1924.     Frequency  of  mutations  for 

chlorophyll-deficient  seedlings  in  maize.     Jour.  Her.,  15:  497-502. 
Hoffer,  G.  N.  and  Holbert,  J.  R.  1918.    Selection  of  disease-free  seed  corn. 

Ind.  A.  E.  S.  Bull.  224. 
Hutchison,  C.  B.,  1922.    Heritable  variations  in  maize.    Jour.  Agron.,  14: 

73-78. 
Jenkins,  M.  T.,  1923.    A  new  method  of  self -pollinating  corn.    Jour.  Her., 

14:  41-44. 
Jones,  D.  F.  1918.     The   effects   of  inbreeding   and   crossbreeding   upon 

development.     Conn.  A.  E.  S.  Bull.  207. 

1920.  Heritable  characters  of  maize,  IV.     A  lethal  factor- 
defective  seeds.     Jour.  Her.  11:   160-167. 

1921.  The  indeterminate  growth  factor  in  tobacco  and  its 
effect  upon  development.      Genetics,  6:  433-444. 

1924.     The  attainment  of  homozygosity  in  inbred  strains  of 
maize.     Genetics,  9:  405-418. 

Kiesselbach,  T.  A.,  1922.     Corn    investigations.        Neb.    A.    E.    S.    Res. 
Bull.  20. 

King,  H.  D.,  1918.     Studies  on  inbreeding,  I-III.     Jour.  Exper.  Zool.  26. 

Lindstrom,  E.  W.,  1920.     Chlorophyll    factors    of    maize.      Jour.     Her. 
11:269-277. 

1923.  Heritable  characters  of  maize  XIII.  Endo- 
sperm defects:  sweet  defective  and  flint  defective.  Jour.  Her.,  14: 
127-135. 

Mangelsdorf,  P.  C,  1923.      The  inheritance  of  defective  seeds  in  maize- 
Jour.  Her.,  14:  119-125. 


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